440 THE ANATOMY OF LWERTEBKATED ANIMALS. 



The parasitic habit which is so rare among the Mollusca 

 occurs in the genus Stylifer, which infests Star-fishes and 

 Sea-urchins, sometimes imbedding itself in the perisoma ; 

 and, under a very remarkable and not yet thoroughly-under- 

 stood form, in the singular parasite of another Echinoderm, 

 Syaapta digitata, termed by its discoverer, Mliller, Ento- 

 concha mirabilis. 1 



In some few of the Synaptce (not more than one, or per- 

 haps two, in a hundred), elongated tubular molluskigerous 

 sacs are found attached by one extremity to one of the intes- 

 tinal vessels ; while the opposite end either hangs freely into 

 the perivisceral cavity, or may be entangled among the bases 

 of the tentacles, at the cephalic extremity of the body of the 

 Synapta. The sac is closed, but, at its attached end, a long 

 invagination extends into its interior. The cavity of the sac 

 beyond the closed extremity of the invagination contains an 

 ovary ; and, beyond this, a certain number of free seminal 

 capsules. The ova are detached from the ovary, and under- 

 go their development in envelopes, each containing many 

 ova, which gradually fill the cavity of the molluskigerous sac. 

 From these ova, embryos, provided with a velum, shell, and 

 operculum, proceed. A large pallial cavity is soon apparent ; 

 but, in the most advanced stages of development observed, it 

 contained no branchiae. 



What becomes of these larvae is unknown, nor is it even 

 certain to what group of the Odontophora Entoconcha be- 

 longs. 



The Pclmonata. — These are odontophorous Mollusks 

 which breathe air directlv, by means of a respiratory surface 

 furnished by the wall of the pallial cavity. 



In some, such as the JPero?iiadce (Fig. 123) and Veronl- 

 cellidce, the body of the slug-like animal is very nearly sym- 

 metrical ; the anus and the lung-sac being situated close to- 

 gether at the posterior extremity of the body. The mantle 

 is large, and extends over the whole haemal or dorsal surface. 

 In all the other Pulmonata, the pulmonary and the anal 

 apertures lie on the right side of the body, and the mantle is 

 provided with at least the rudiments of a shell. The pallial 

 region is sometimes very small in proportion to the rest of 

 the body, and then forms a flattened disk, as in the common 

 Slug ; while, in some Limacidce and Testacellidce, and in the 



1 "Die Erzeuerung von Schnecken in Holothurien,'" 1352. Baur, " Ueber 

 Synapta digitata^ (" Nova Acta," xxxi., 1864.) 



