320 INVERTEBRATA CHAP. 



of the other Gastropoda which have been studied. We find that those 

 primitive forms which preserve the original bilateral symmetry (the 

 Polyplacophora, Chiton and its allies) possess a typical Trochophore 

 larva, similar in all respects, so far as our knowledge goes, to that of 

 Patella. 



The cell-lineage of Ischnochiton, as worked out by Heath (1899), 

 seems to be exactly similar to that of Patella, and the formation 

 of organs in the European Chiton polii, as described by Kowalevsky 

 (1883), wears an even more primitive aspect than in Paludina ; for 

 example, the two pericardial sacs in Chiton polii are large, and occupy 

 most of the post-trochophoral region. A re-examination of the later 

 larval history of Polyplacophora by the aid of refined modern methods 

 would be of rare interest. 



A Trochophore larva is also found in the case of such primitive 

 forms as Trochus, which has been worked out in great detail by 

 Robert (1902), and in Acmaea, Fissurella, and Haliotis, all of which 

 retain two auricles in the heart and two kidneys. 



In all other Gastropoda, so far as is known, the embryo becomes a 

 larva only when the Trochophore stage has been passed through and 

 the coiled shell has been formed. Since, in almost every case, the 

 eggs only develop after they have been laid, this involves their being 

 laid in capsules secreted by the oviduct of the mother. Sometimes 

 many eggs are contained in a capsule (Prosobranchiata generally), 

 sometimes only one (Opisthobranchiata and Pulmonata), and in this 

 latter case the capsules are very small and generally embedded in a 

 jelly which is difficult to get rid of. The capsules of the Proso- 

 branchiata are usually large, they are unprotected by jelly and 

 attached singly to submarine objects such as stones. It is easy 

 enough to detach them and slit them open, and in this way a supply 

 of embryos can readily be obtained. 



It is interesting to see in some such cases (cf. Purpura) the 

 beginnings of the same process which has led to such distortions of 

 development as are seen in the case of Platyhelminthes. Some of the 

 embryos in a capsule develop imperfectly and are swallowed by their 

 successful sisters, to whom they serve as pabulum. 



In dealing with the egg-capsules of Pulmonata and Opisthobranchi- 

 ata, which are immersed in jelly, various methods are employed. 

 Sometimes, as in the case of the Opisthobranch Fiona, it is found 

 possible to use reagents (for example, picro-acetic and picro-sulphuric 

 acids) which will preserve the whole mass in bulk, and the jelly with 

 the contained egg-capsules can then even be embedded in paraffin ; 

 but usually it is necessary to remove both jelly and capsules. 



We may give Wierzejski's method of dealing with the eggs of 

 Physa (1905) as a good example of how this can be accomplished. 

 The egg-capsules are dissected out of the jelly by needles, then 

 they are immersed for two or three minutes in a mixture of 

 the solution of corrosive sublimate in water and glacial acetic 

 acid, and the adhering jelly is then removed by the action of 



