The remainder of the body of Actinotrocha corresponds with the trunk of Balanoglossus. Its body-cavity is distinct from that of the collar, and is divided by a ventral mesentery, though not by a dorsal mesentery. A noteworthy fact is that both Actinotrocha and Tornaria swim by means of a ring of strong cilia or membranellae[[52]] which surrounds the anus.
Fig. 14.—Actinotrocha-larva of Phoronis. a, Anus; b.c1, b.c2, b.c3, first, second and third body-cavities; c, circular nerve, running in the posterior boundary of the collar, immediately behind the ring of tentacles; c.r, ciliated ring; d, diverticulum (paired) of alimentary canal; m, mouth; n.s, central nervous system; p, nerve running round the ventral border of the proboscis; s, sense-organ; s.s, subneural sinus, a vascular space whose hind wall is constituted by the front boundary of b.c2, its front wall being formed by the hind wall of b.c1; in this region is seen a median outgrowth of the alimentary canal, which may be compared with the notochord of Cephalodiscus, or of the young Tornaria (cf. Morgan, J. Morphol. v. 1891, Plate xxvi. Fig. 40.) (After Masterman.)
Important memoirs on the structure of Actinotrocha have recently been published by Ikeda,[[53]] de Selys Longchamps,[[54]] Goodrich,[[55]] and Schultz,[[56]] who criticise many of Masterman's statements. While it is admitted on all sides that an oblique septum following the line of the bases of the tentacles completely subdivides the body-cavity, Masterman's account of the anterior cavities is not confirmed, the spaces indicated by b.c1 and b.c2 in Fig. 14 being stated to be really continuous with one another, while the "subneural sinus" (s.s) is regarded as a part of this space. It appears, however, from the account given by Ikeda, and followed by Goodrich, that the old Actinotrocha has two distinct spaces in front of the septum. The first of these corresponds with b.c1 + most of b.c2 in Fig. 14, and is continuous with the cavities of the larval tentacles. Into it project the blind ends of the larval excretory organs, which, according to Goodrich, bear numerous "solenocytes" similar to those described by the same author in Amphioxus and in Polychaet worms (Fig. 79, p. [127]). The second cavity is a relatively small crescent (not shown in Fig. 14), lying on the anterior face of the septum, the tips of the crescent nearly meeting dorsally, so as to constitute an almost complete ring following the bases of the tentacles, into each of which it gives off a blind outgrowth. At the metamorphosis, the crescentic space becomes the prae-septal body-cavity and the cavities of the tentacles of the adult, the circular blood-vessel of which is formed from the remains of the large prae-septal space of the larva. Schultz, in calling attention to the fact that both Phoronis and its larva have a striking power of regenerating lost parts, confirms the conclusion that this animal belongs to the Hemichordata. He gives reasons, however, for believing that it is in the adult Phoronis rather than in the larval Actinotrocha that it is possible to discover the most satisfactory evidence of this affinity.
The metamorphosis[[57]] of Actinotrocha is very remarkable, and is accompanied by the eversion of a ventral ingrowth of the body-wall. A loop of the alimentary canal passes into this eversion, which becomes the main part of the body of the adult; and the anus is thereby brought relatively nearer the mouth than in the larva. The occurrence of this process may help to explain the position of the anus in the Pterobranchia.
Affinities of the Hemichordata.—There can be no doubt that some of the resemblances, in structure and in development, between Balanoglossus and certain Vertebrates are extremely striking. The view that Balanoglossus is related to the ancestors of Vertebrates[[58]] appears to exclude other views[[59]] which have been suggested with regard to the same question. The Balanoglossus-theory does not explain the similarity between the segmentation and the excretory systems of Vertebrates and Chaetopods; but, on the contrary, there are important characters which Vertebrates share with Balanoglossus but with no other "Invertebrates." Of these the most important appear to be the resemblances between the gill-slits and gill-bars of Balanoglossus and Amphioxus; the position, structure and mode of development of the central nervous system; and the presence of a structure in the Hemichordata, which may be regarded as the notochord. There are other points in which Balanoglossus specially resembles Amphioxus, such as the early development, the mode of formation of the body-cavities,[[60]] and the presence of numerous generative organs.
All these, taken together, make it necessary to consider carefully the claims of Balanoglossus to relationship with the ancestors of Vertebrates in making any speculations on this interesting problem.
However improbable it may appear at first sight, it is possible to hold the view that Balanoglossus is related at the same time to Vertebrates and to Starfishes and other Echinoderms. The similarity between a young Tornaria and a young Bipinnaria-larva of a Starfish is so great as to have misled even Johannes Müller. The more obvious resemblances are the almost identical course of the longitudinal ciliated band in the young stages, and the presence of a dorsal pore. The Echinoderm-larva is not, however, provided with eye-spots, nor has it the posterior, or transverse, ciliated band of Tornaria.
Recent studies on the development of Echinoderms[[61]] have made it probable that the five body-cavities of Balanoglossus are represented in the larvae of those animals; and this materially strengthens the probability of the view that the respective adults are also allied.[[62]] It may be added that the relationship which appears to be indicated is between Balanoglossus and the bilateral ancestors from which the radially-symmetrical Echinoderms are probably descended.
In comparing the Enteropneusta with the Pterobranchia, the disproportionate size of the trunk of Balanoglossus may perhaps be explained by assuming that the region of the third body-cavities has been enlarged since Balanoglossus branched off from the ancestral stock.[[63]] The approximation of the anus to the mouth in Pterobranchia is perhaps the result of their tubicolous habits.[[64]] In the position of the central nervous system in the skin of the collar, Cephalodiscus appears to be more primitive than Balanoglossus, as has been pointed out by Morgan.[[65]] It is not impossible that the presence of one pair of gill-slits in Cephalodiscus indicates that this animal diverged from the ancestors of Balanoglossus before the gill-slits were metamerically repeated.