THE EVIDENCE FROM DEVELOPMENT. 



229 



FIG. 153. 

 LARVAL, OR 



YOUNG 

 PTEROPOD. 



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 ^Lolis (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 pteropbd 



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. 



