MOLLUSCS. 



297 



<'i 



I 



li;iving its ])L'culiar mode of oviposition. Tlie esxgs aiv imbedded in n, transparent 

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



Nearly 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 Enioconcha niira- 

 bilis, which leads a parasitic life 

 inside the body of iSi/iHq)ta, one of 

 the holothurians. So greatly has 

 ]iMrasitism altered the form of the 

 body, and all of the organs, that 

 the proj)er ]>osition of this form 

 among the gasteropods is far from 

 certain, some placing it near JVcitica. 

 Indeed, were it not for the charac- 

 ters afforded by the young, its posi- 

 tion among the mollusea would not 

 be susjieeted. Some thirty years 

 ago Johannes Midler found in some 

 s])ecimens of Synapia digituta an 

 internal worm -like ]iarasite, at- 

 tached by one extremity to the ali- 

 mentary canal, while the other end 

 hung free in the j)erivisceral cavity. 

 Other observers, notably Baur, have 

 investigated this strange form, bnt 

 there are many facts concerning it 

 yet to be ascertained. 



In about one sjiecimen of Sipi- 

 apta out of one or two hundred 

 this strange form occurs. It is a 

 sac, the ujiper jwrt 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, ])roducing a 

 velum, shell, and ojiereulum, the 

 later stages being found free in the 

 body-cavity of the host. After the stage shown in Fig. 883, nothing more is known of 

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

 from the characters of the embryo, that Entoconclm should possibly l)e assigned a jdace 

 among the nudibranchs. A second species of Entoconcha ( E. Diillkri) is found in Ifolo- 

 tlmria edulis. in the Philipi)ine8. 



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. 335.— .4, Si/iiapta ilitiitatn, with parasitic Kiitarnncha, natural 

 size; B, a portion of 's>/napta witli EtifncoJicha (F) enlarged; «, 

 point of attachment; /', blooil vessels; f, female portion; i, in- 

 testine; 111, male portion; we, mesentery. 



