248 COMPARATIVE ANATOMY CHAP, vn 



filled with embryos (not represented in the figure), has been observed. In a widening 

 of the tube near its attached end, a number of free " testicular vesicles " have been 

 found, but their real significance can only be discovered by further research. 



The embryos found in the brood cavity of Entoconcha have the same general 

 structure as Gastropodan larvae. They have a spirally twisted shell, into which 

 the body can be withdrawn ; an operculum, a small velum, the rudiments of two 

 tentacles, two auditory vesicles, [a foot, and an intestine, which, according to one 

 observer (the most recent), consists of only a mouth, pharynx, oesophagus, and the 

 rudiment of a liver, but according to an older authority is complete. There is, 

 further, a branchial cavity with a transverse row of long cilia. Nothing further is 

 known of the development and life history of Entoconcha. 



Some details of parasitic Lamellibranchiate larvae (Unionidoc) will be given in the 

 section on Ontogeny. 



XXII. Attached Gastropoda. 



Of the several forms of attached Gastropods known, only Vcrmetus, whose inner 

 organisation has been carefully investigated, can be shortly described in this place. 

 Vermetiis has a shell which, instead of being coiled like the well-known shell of the 

 snail, is a calcareous tube, which rises freely from the bottom of the sea, to which its 

 tip is cemented. This shell is very like the calcareous tubes of tubicolous worms 

 such as Serpula. The larva of this form, however, possesses a typically coiled shell, 

 and even the young animal, after it has attached itself, has such a shell. In the 

 course of growth, however, the coils become loosened, and the shell finally grows out 

 as a tube. 



The typical organisation of the Monotocardian Prosobranchiates, to which Vermetus 

 belongs, is little affected by the attached manner of life. The visceral dome, like 

 the shell, is much elongated and almost vermiform. The intestine, the circulatory 

 system, the kidneys, the mantle, the gill, and the nervous system are typically 

 developed. The sexes are separate, and copulatory organs, which could not be 

 used by attached animals, are wanting. The head is well developed, and the 

 pharynx well armed. When the animal is slightly irritated, it is said not to 

 withdraw at once into its shell, like other Gastropods, but to bite. The foot has 

 the form of a truncated cylinder, and is directed anteriorly, ventrally to the head. 

 It cannot, of course, function as a locomotory organ, but carries the operculum for 

 closing the shell, and, by means of the pedal gland, secretes mucus. Venmtus is 

 said to produce great quantities of this secretion, which it allows to float in the 

 water for a time like a veil, and then swallows together with all that has become 

 attached to it. In this way it fishes for the small organisms which form its food. 



XXIII. Ontogeny. 



A. Amphineura. 



1. Ontogeny of Chiton Polii (Fig. 207). The egg possesses little nutritive yolk. 

 The segmentation is total and somewhat unequal ; a ccelogastrula is formed by 

 invagination. 



(a) The blastopore of the gastrula larva marks its posterior end. A pair of 

 endoderm cells near the dorsal edge of the blastopore are specially large. A 

 longitudinal section shows two dorsal and two ventral ectodermal cells with larger 



