Fig. 225.—A, Epithetosoma norvegicum K. and D., magnified. a, a, Right and left slits leading to the pores; b, mouth; c, proboscis: B, the same animal opened dorsally; a, pores; b, oesophagus; c, proboscis; d, brown tube. (After Danielssen and Koren.)
Affinities of the Gephyrea.
Before considering to what other groups of animals the Gephyrea may be allied, it is advisable to discuss the relationship of the four Orders which compose the group.
Quatrefages, in the year 1865, divided the Gephyrea into I. Gephyrea Armata, with which he included the Echiuroidea and Sternaspis,[[489]] and II. Gephyrea Inermia or Sipunculoidea. The Gephyrea Inermia, sometimes called the Achaeta, have been extended to include the Order Priapuloidea, and opposed to the smaller sub-group the Gephyrea Armata or Chaetifera. In my opinion, however, these names now are no longer in accordance with our knowledge of the structure of the animals they attempt to describe, and they should be given up. Both names had reference to the presence or absence of the two hook-like bristles described on the ventral surface of some of the Echiuroidea, but of the five genera of this family, two, Saccosoma and Hamingia (the latter in the female or normal form), are without these bristles, and can therefore be described neither as Armata nor as Chaetifera. On the other hand, hook-like chitinous bristles of somewhat the same nature, though smaller in size and varying in position, are very common on the introvert of Sipunculoidea and on the body of the Priapuloidea.
Again, the association of the two last-named Orders in one sub-group is, to my mind, an error. The Priapuloidea have little in common with the Sipunculoidea; almost the only real point of resemblance is the power of protruding the anterior part of the alimentary canal, and withdrawing it by the aid of retractor muscles. But in the Priapuloidea this power exists to a very small extent, and it is a power shared by very many animals besides the Gephyrea. The terminal anus of the former is a feature shared by the Echiuroidea and by Epithetosoma, but these have little else in common with the Priapuloidea. On the other hand, the entire absence of any head appendages, such as the proboscis of the Echiuroidea and the tentacles or tentacular membrane of the Sipunculoidea, the absence of a vascular system, of nephridia or anal vesicles, taken together with the straight intestine which occurs elsewhere only in Epithetosoma, the persistent connexion of the nervous system with the epidermis, the unique character of their excretory system and of the reproductive organs, are all features in which the Priapuloidea differ from the more normal members of the other three Orders. These constitute a list of peculiarities which are at least as important, and probably even more important, than those which characterise the Sipunculoidea and the Echiuroidea. Thus the Priapuloidea should, I think, be regarded as a distinct Order, which occupies a very isolated position in the group.
Until we know something about the development of Halicryptus and of Priapulus, it will be difficult to say whether the Order is more nearly allied to one or the other of the two great Orders of Gephyrea, whether it is very primitive or very specialised. The connexion of the entire nervous system with the epidermis and the absence of a vascular system are both rather primitive features, and so is the Platyhelminthine character of the excretory organs. With regard to the vascular system, however, it should be pointed out that it arises very late in the larva of those Gephyrea whose development is known, and that it does not seem to correspond with the vascular system of other animals; it has no fine vessels or capillaries connected with it, and apparently does not act so much as the channel of the circulatory medium, but more as a mechanism for the expansion of the head appendages, the tentacles in the Sipunculoidea and the proboscis in the Echiuroidea; moreover, it is absent in some genera of the former, such as Onchnesoma, Tylosoma, and Petalostoma, where there are no tentacles.
The conclusion of the whole matter seems to be that the Priapuloidea are an isolated Order retaining many primitive features, and having no closer affinities to the Sipunculoidea than to the Echiuroidea.
Hatschek came to the conclusion, from his work on the development of Echiurus, that the Echiuroidea are true "Annelids," and from the presence and mode of formation of the bristles, that they are related to the Chaetopods. In this view he is confirmed by Conn, who worked out the development of Thalassema. This relationship is further confirmed by the discovery of Sluiter's that Sternaspis, the genus of Chaetopods which in other respects most nearly resembles the Gephyrea, has in one of its species (S. spinosa) a well-marked bifid proboscis, which, like that of the Echiuroidea, is thrown off at the least disturbance. Thus it seems fairly well established that the Echiuroidea are closely connected with the Chaetopoda, for although the only traces of segmentation they retain in the adult are the serially-repeated nephridia of Thalassema and Echiurus pallasii, and the two rows of peri-anal bristles in the latter, and possibly the circular nerves given off from the ventral cord, yet the larva is fully segmented, and in other respects is almost typically Chaetopodan.
The relationship of the Sipunculoidea to the Echiuroidea is a more doubtful point. Hatschek is inclined to separate them, and in this he is again supported by Conn. Embryology unfortunately does not help us much. The early stages and larvae of Sipunculus nudus and of Phascolosoma elongatum have been investigated by Hatschek and by Selenka respectively. In neither genus is there any trace of segmentation or of Annelid features, with the possible exception of the bristles on the larval Phascolosoma. On the other hand, it must be remembered that the development of Sipunculus is remarkably abbreviated, and that such stages may have dropped out, the larvae hardly differing more from the Trochosphere of Echiurus and Thalassema than does that of Bonellia, an undoubted Echiurid. Still the facts that there is never a head-kidney present, that there is no trace of segmentation, and that at no stage is the anus terminal, must have a certain weight.
If we leave out of account the larval history, which, although pointing to a difference in the nature of the two families, is by no means decisive, and consider the adult structures, we find very considerable evidences of affinity. Taking firstly the main points of difference, we find these to be (i.) the nature of the cephalic appendages, either a proboscis or some modification of tentacles; (ii.) the position of the anus; (iii.) the presence of anal vesicles; (iv.) the number of the nephridia, never more than one pair in Sipunculids; and (v.) the difference in origin of the chaetae. Of these most undoubtedly the first is the most important. The Echiuroidea have retained the prae-oral lobe of the larva in the form of a solid outgrowth of the body, which outgrowth has carried with it the nerve-ring and vascular ring which surround the mouth. This has been lost in the Sipunculoidea, but is, I think, represented by a modified patch of epidermis which lies dorsal to the mouth and just above the brain. A solid extension of the skin in this region, which involved the nervous and vascular systems, would bring about the same relation of parts as is found in the Echiuroidea. The tentacular membrane or tentacles of the Sipunculoidea have such a variety of form and arrangement, whilst all subserving the same end, that I am inclined to believe that they have originated within the limits of the family.