THE EVIDENCE FROM DEVELOPMENT. 



229 



paddles or flaps its way through the sea by means of a pair of wing-like 

 fins attached to the sides of the neck. Such are the " Sea-butterflies," 

 or Pteropoda (Figs. 140 and 152, B), already mentioned as a class of the 

 Molluscan group. Their title to be regarded as " shellfish " rests on 

 the fact that, besides agreeing with other molluscan characters, they 

 may possess a delicate glassy shell (Fig. 140, C); but this structure may, 

 at the same time, be wanting, and a head may also be indistinctly 

 represented the latter fact indicating, as we have seen, a 

 position of inferiority in the molluscan scale. Now, when 

 a Pteropod (Fig. 152, B) is even cursorily regarded in the 

 possession of its "wings" or fins, borne on the sides of its 

 neck, its resemblance to the young (Fig. 152, A) of some 

 of the "naked" gasteropods, such as JEolis (Fig. 150), 

 is both close and unmistakable. In their development 

 the pteropods possess a "velum," like most univalves. 

 This " velum " is believed by good authorities to remain 

 developed, and to constitute the "wings" or "fins" 

 (Fig. 140, A, a) of these animals. By other authorities 

 their " fins " are believed to represent certain side-lobes 

 of the molluscan body, and as such are regarded by this 

 second theory as secondary developments. However, 

 that the pteropods represent a rudimentary or primitive set of beings 

 no one may doubt. Let us bear in mind that they run through the 

 same early phases of development as gasteropods, and that not only 

 is the "velum" or " veliger- 

 stage" represented in their 

 history, but that certain 

 members of their class 

 present the cilia-girdled 

 appearance (Fig. 153) 

 proper to the early phases 

 of worm development 

 (Fig. 157). Let us also 

 reflect that the pteropod 

 seems to have been ar- 

 rested in its development 

 at, or a little beyond, the 

 " veliger- stage," and we 

 may readily understand the 

 position of those naturalists 

 who, comparing the young 

 of the " naked "gasteropod 



(Fig. 152, A) with the adult pteropod (B), see the closest affinity 

 and relationship between them. The pteropod in this view repre- 

 sents a " permanent larval " or arrested gasteropod. Both have 



FIG. 154. BRACHIOPODA AND DEVELOPMENT. 



