96 A. E. Verrill — Molluscan Archetype. 



organs of the proveligers and subveligers of these diverse 

 groups can be strictly compared, part for part. Their princi- 

 pal differences consist in the form of body ; in the shell when 

 it appears, and especially in the degree of development of the 

 velum. This larval locomotive organ may be, as in bivalves, a 

 simple, thickened, ciliated ridge around the cephalic region 

 (fig. 18) ; it may develop two broad rounded ciliated lobes, as 

 in most gastropods (figs. 6, 9) ; or these lobes may become sub- 

 divided into four or more lobes, as in Natica (fig. 10), and 

 certain pteropods. 



These molluscan larvae, in an earlier stage of development, 

 closely resemble the corresponding larvae of certain annelids ; 

 they also resemble in several respects certain adult Kotifera. 

 The first distinctive molluscan organ that appears is the shell- 

 gland (figs. 12, 17, s\ which is formed by an invagination, at 

 the posterior or postero-dorsal end.* Its universal presence in 

 the larvae, even of such species as never have a shell, is very 

 significant. The 'digestive system at this time may consist only 

 of a simple cavity (stomodeum) and the mouth {a). The latter 

 is situated an tero- ventral ly ; it is usually, but not always, 

 formed from the blastopore. f The anterior or cephalic region 

 {a'p) is differentiated at an early period, and carries a preoral 

 circle of cilia representing the rudiment of the velum {v). 

 The anus {e) is nearly always formed somewhat later than the 

 mouth, by an invagination. At first it is usually only a little 

 way back of the mouth. Its completion marks the true veliger 

 stage. The foot begins in the proveliger as a slight ventral 

 protrusion {f\ between the mouth and anus, and as it grows 

 these two openings are pushed farther and farther apart ; they 

 may finally arrive even at opposite ends of the body, as in the 

 bivalves. The shell {s\ beginning at the shell-gland, appears 

 quite early in the subveliger stage, and soon spreads over 

 much of the body, and at once shows by its form and 

 structure to which class the veliger belongs, for in the sub- 

 veligers of Bivalvia:}: the shell is, from the first, bivalve 

 (fig. 18) ; in that of the Scaphopoda it is saddle-shaped or sub- 

 tubular (fig. 16, 5) ; in that of the Gastropoda it is hemispherical 

 or cup-shaped and usually more or less incurved,§ subspiral, or 



* It is not improbable that the secretion of the shell-gland was used as the first 

 means of adhesion in many primitive forms of cephalopods and gastropods, 



f According to the figures and descriptions of observers, the blastopore some- 

 times becomes the anus, sometimes elongates and then divides to form both 

 mouth and anus, but in much the largest number of groups it forms the mouth 

 alone. 



X The name Bivalvia, given to this group by Linne, should replace the various 

 later and less characteristic names, such as Lamellibranchiata, Pelecypoda, Con- 

 chifera, Acephala, etc, \ 



§ The primary form was probably hemispherical or cup-shaped, a form still 

 found in the veligers of many pteropods (figs. 13, 14). The first curvature or 

 incurvature of the shell was probably due to the pulling action of the strong 

 retractor muscles of the velum. This is very evidently the case in pteropod veli- 

 gers in which the young shell is bent only a little, but precisely in the direction 



