DEVELOPMENT OP THE EXTERNAL FORM PTEROPODA. 167 



(velum, mouth, shell-gland, foot, etc.) appear, varies greatly in different forms. 

 The comparison of corresponding stages is in this way rendered somewhat 

 more difficult, but the final result is, as already stated, very similar. 



The embryonic development of a large number of Pteropoda (Cavolinia 

 (Hyalen), ///A//V///?.r, Creseis, Stijliola, Cleodora, Cymbulia, Clione) has been 

 closely studied by FOL, who has also described the further development and 

 the metamorphosis of these animals (No. 32). The phenomena connected 

 with metamorphosis had previously been investigated especially by JOH. 

 MULLER, GKGENBAUR, and KROHN in the above genera as well as in Tiede.mannia 

 and Pneumndcnno), (Nos. 77-79, 37 and 58a). 



Thecosomata. The Tr<><-lio/>h<n'e stage soon passes into the Veliger 

 stage, a dorsal and a ventral incision appearing in the velum, which 

 thus becomes bilobed. This organ is bordered anteriorly by a circle 

 of strong cilia serving for locomotion, and posteriorly by weaker cilia 

 which conduct food to the mouth 

 (GEGENBAUR, FOL). In Cleodora 

 a band of cilia appears on the velar 

 area at a time when the larva is 

 still at the Trochnphon'. stage. 

 Other Pteropods, e.y., Cavolinia, 

 carry on the velar area a central 

 ciliated tuft, such as has been met 

 with in other Molluscan larvae. 



The size of the velum varies 

 greatly. In Cavolinia, where it 

 is retained for only a short time, 

 it is less extensive. In Fig. 74, 

 we see the velum in a slightly 

 older larva of such a form. In 

 Cleodora, Cymbulia, Tiedemanuia 

 (Fig. 75 A and B) the velum is 

 much larger, and each of the two 

 lobes is again subdivided, so that 

 the whole appears to consist of 

 four lobes. This condition is 

 specially distinct in a larva be- 

 longing to the genus Creseis and 

 described by GEGENBAUR (Fig. 

 7.^ (?), in which the velum is still 

 of considerable size when the shell has grown to a great length. A 

 strong retractor starts from the anterior part of the body and is 

 inserted at the posterior end of the shell (Fig. 76 A, r). 



mr 



FIG. 74. Larva of Cavolinia tridentata, 

 seen from the right and ventral side 

 (after FOL, from BALFOUR'S Text- 

 book), a, anal region, with the two 

 anal cells ; /, mesopodium ; h, heart ; 

 /, intestine ; kn, contractile dorsal 

 sinus ; m, oral region ; mb, mantle ; 

 me, mantle-cavity ; ot, otocyst ; pn, 

 rudiment of fin ; q, shell ; r, renal sac ; 

 s, stomach > a, food-yolk. 



