MOLLUSCS. 



297 



having its peculiar mode of oviposition. The eggs are imbedded in a transparent 

 gelatinous matrix, allowing the earlier stages of development to be readily seen. 



Kearly one thousand species of nudibranchs have been described, from all seas ; but 

 as these forms have not been studied to the same extent as their shelled relatives, this 

 number will doubtless be greatly in- 

 creased by subsequent researches. 



The first form requiring notice 

 is the peculiar Entoconcha mira- 

 bilis, which leads a parasitic life 

 inside the body of Synapta, one of 

 the holothurians. So greatly has 

 parasitism altered the form of the 

 body, and all of the organs, that 

 the proper position of this form 

 among the gasteropods is far from 

 certain, some placing it near Ncitica. 

 Indeed, were it not for the charac- 

 ters afforded by the young, its posi- 

 tion among the mollusca would not 

 be suspected. Some thirty years 

 ago Johannes Milller found in some 

 specimens of Synapta digitata an 

 internal worm - like parasite, at- 

 tached by one extremity to the ali- 

 mentary canal, while the other end 

 hung free in the perivisceral cavity. 

 Other observers, notably Banr, have 

 investigated this strange form, but 

 there are many facts concerning it 

 yet to be ascertained. 



In about one specimen of Syn- 

 apta out of one or two hundred 

 this strange form occurs. It is a 

 sac, the upper part bearing the 

 female and the lower the male re- 

 productive organs, while the cen- 

 tre of the body serves for a while 

 as a brood pouch, the embryos later 

 passing out from an opening at the 

 free end of the body of the parent. 

 The eggs undergo a tolerably reg- 

 ular development, producing a 

 velum, shell, and operculum, the 

 later stages being found free in the 

 body-cavity of the host. After the stage shown in Fig. 333, nothing more is known of 

 their history. It would appear, from the little that is known of the development, .and 

 from the characters of the embiyo, that Entoconcha should possibly be assigned a place 

 among the nudibranchs. A second species of Entoconcha {E. miXlleri) is found in Holo- 

 thuria eduUs, in the Philippines. 



Fio. 335. —A, Synapta cliyitata, with parasitio Entocnncha, natural 

 size; B, a portion of Synapta with Entoconclm (F) enlarged; a, 

 point o£ attaoliment; b, blood vessels; /, female portion; i, in- 

 testine; 7?!, male portioa; me, mesentery. 



