152 



GASTROPODA. 



other Gastropod larvae. Nassa now shows a strong general resem- 

 blance to such larvae, as is evident from Fig. 61 E. This is also the 

 case with Fums, the embryos of which also at first deviate in 

 several points from the usual shape and resemble those of Nassa.] 



Bobretzky has described in connection with Nassa and Fusus an organ of 

 which no account has as yet been given ; this is the so-called " larval heart,' 7 

 which has also been found in other Gastropoda {e.g., by Salensky in 

 Calyptraea, No. 98). This larval heart (Fig. 61 E, Ih) is said to be a part of 

 the ectoderm lying dorsally behind the velum, which is connected with 

 mesodermal elements and carries on contractile movements. Other parts of 

 the embryo, such as parts of the cephalic vesicle and the foot, are said to be 

 capable, like this region, of contractile movements. 



Cr.v 



Fig. 63. — Longitudinal section through an 

 embryo of Nassa m utabilis at a slightly 

 older stage than in Fig. 61 D (after Bo- 

 bretzky from Balfour's Text-book). 

 The cephalic section and the foot of the 

 embryo have separated to a great extent 

 from the yolk which forms the posterior 

 part of the embryo, ce.v, cephalic vesicle ; 

 /, foot ; hi, mouth ; st, stomach. 



Remarkable transformations 

 in the shape of the body take 

 place in some Prosobranchs 

 which become adapted to a 

 parasitic life on or in various 

 Echinoderms (Asteroids, Echi- 

 noids and Holothuroids). An 

 excellent example of this is 

 afforded by Entoconcha miralrilid 

 described by Joh. Mltll.ee 

 (No. 76,) as occurring in the 

 body-cavity of Synapta digitata 

 attached to the wall of the 

 intestine. The body of this 

 animal has the form of a long 



vermiform coiled tube which 

 in no way recalls that of a 

 Gastropod, but its brood-cavit} T contains embryos very like those 

 of other Prosobranchs. These have a velum (not, it is true, very 

 highly developed), a spirally coiled shell, a foot with an operculum, 

 otocysts, etc. Their further development is not known, but it is 

 probable that they live freely for a time, like the young Entovalva 

 (p. 13), and only later wander into a Holothurian. 



In explaining the remarkable transformation undergone by Ento- 

 concha in consequence of its parasitic life, two Prosobranchs described 

 by P. and F. Sakasin {Thyca tntoconcha and Stilifer Linckiae) are of 

 great value (No. 103). These forms live parasitically on Asteroids, 

 either piercing the integument by means of a proboscis-like structure 

 {Thyca) or else sinking bodily into it {Stilifer). Even these ecto- 



