CHAPTER XV

GEPHYREA

INTRODUCTION—ANATOMY—DEVELOPMENT—SIPUNCULOIDEA—PRIAPULOIDEA—ECHIUROIDEA—EPITHETOSOMATOIDEA—AFFINITIES OF THE GROUP.

The animals included in the above-named group were formerly associated with the Echinodermata. Delle Chiaje[[468]] states that Bohadsch of Prague in 1757 was the first to give an accurate description of Sipunculus under the name of Syrinx, but Linnaeus, who noted that in captivity the animal always kept its anus directed upwards, re-named it Sipunculus. Lamarck[[469]] placed the Gephyrea near the Holothurians; and Cuvier[[470]] also assigned them a position amongst the Echinoderms. He mentions Bonellia, Thalassema, Echiurus, Sternaspis, and three species of Sipunculus, one of which, S. edulis, "sert de nourriture aux Chinois qui habitent Java, et qui vont la chercher dans le sable au moyen de petits bambous préparés."

The name Gephyrea[[471]] was first used by Quatrefages, who regarded these animals as bridging the gulf between the Worms and the Echinoderms. He included in this group the genus Sternaspis (vide p. [335]), now more usually classed with the Chaetopoda.

The Gephyrea are exclusively marine. They are subcylindrical animals, which can either retract the anterior end of their body—the introvert—carrying the mouth into the interior; or are provided with a long flexible but non-retractile proboscis. The latter is easily cast off. They usually bear spines or hooks of a hard chitinous character, secreted by the epidermis or outermost layer of cells. The mouth is at the base of the proboscis or at the end of the protractile part, the anus is at the other end of the body or on the dorsal surface. The nervous system consists of a ring round the mouth and of a ventral nerve-cord. A vascular system is present as a rule. Nephridia are found which act as excretory organs, and in most cases also as ducts for the generative cells. The Gephyrea are bisexual, and the male is sometimes degenerate.

The group may be divided into four Orders:—(i.) Sipunculoidea; (ii.) Priapuloidea; (iii.) Echiuroidea; (iv.) Epithetosomatoidea; of these the first is by far the largest, both in number of genera and of species.

The Anatomy of Sipunculus nudus.

External Characters.—The body of S. nudus when fully extended may attain a length of a foot, or even a little more; in this condition it is seen to consist of two portions, the anterior of which is, however, retracted into the other when the animal is disturbed. The retractile portion is sometimes termed the proboscis, but as its nature is entirely different from that of the proboscis of the Echiuroidea, it is better to refer to it as the introvert. Special retractor muscles are attached on the one hand to the body-wall about half-way down the body, and on the other hand are fused into a muscular sheath which surrounds the gullet, just behind the mouth. When these muscles contract, they withdraw the introvert into the rest of the body or trunk in much the same way as the finger of a glove may be drawn into the hand, by a thread fastened to the inside of its apex. The introvert is protruded by the contraction of the circular muscles of the body-wall. These exert a pressure on the fluid which fills the body-cavity, and by this means the sides of the introvert are forced forward until finally the head is exposed.

The introvert occupies about one-sixth or one-fifth of the total body length. It is somewhat narrower than the trunk, and is covered by a number of small flattened papillae, some of which lie with their free ends directed backward, overlapping one another like tiles on a roof. In some other genera, as Phymosoma, the introvert bears rows of horny hooks, which are apt to fall off as the animal grows old.