80 ESSAY ON CLASSIFICATION 



the proles polypoidea of the Acalephs is as completely an Acaleph 

 as their proles medusina, and whether they separate or remain con- 

 nected, their structural relations are everywhere the same. A com- 

 parison of Hydractinia, which is the most common and the most 

 polymorphous Hydroid, with our common Portuguese Man-of-War 

 (Physalia) may at once show the homology of their most polymor- 

 phous individuals. 



The embryology of Mollusks has been very extensively investi- 

 gated, and some types of this branch are among the very best known 

 in the animal kingdom. The natural limits of the branch itself ap- 

 pear, however, somewhat doubtful. I hold that it must include the 

 Bryozoa, which lead gradually through the Brachiopods and Tuni- 

 cata to the ordinary Acephala, and I would add that I have satisfied 

 myself of the propriety of uniting the Vorticellidas with Bryozoa. 

 On the other hand, the Cephalopods can never be separated from 

 the Mollusks proper as a distinct branch; the partial segmentation 

 of their yolk no more affords a ground for their separation than the 

 total segmentation of the yolk of Mammalia would justify their sepa- 

 ration from the other Vertebrata. Moreover, Cephalopods are in all 

 the details of their structure homologous with the other Mollusks. 

 The Tunicata are particularly interesting, inasmuch as the simple 

 Ascidians have pedunculated young which exhibit the most striking' 

 resemblance to Boltenia and form at the same time a connecting 

 link with the compound Ascidians. The development of the La- 

 mellibranchiata seems to be very uniform, but they differ gTeatly as 

 to their breeding, many laying their eggs before the germ is formed, 

 whilst others carry them in their gills until the young are entirely 

 formed. This is observed particularly among the Unios, some of 

 which, however, lay their eggs very early, while others carry them for 

 a longer or shorter time in a special pouch of the outer gill, which 

 presents the most diversified forms in different genera of this family. 

 Nothing is as yet known of the development of Brachiopods. The 

 Gasteropods exhibit a much greater diversity in their development 

 than the Lamellibranchiata. Even among the terrestrial and aquatic 

 Pulmonata there are striking differences. Some of the Pectinibran- 

 chiata are remarkable for the curious cases in which their eggs are 

 hatched and the young developed to an advanced state of growth. 

 The cases of Pyrula and Strombus are among the most extraordinary 



