318 COMPARATIVE ANATOMY. 



Pteropoda i Esciiricht, Ueber d. Clione borcalis. Kopenhagen, 1838.— Fol, H., Sur le tle"voloppe- 

 nient cles Pteropodes. Archives tie zoologie. T. IV. 



Cephalopoda : Grant, On Loligopsis. Trans. Zool. Soc. 1835. — Owew, Memoir on the Pearly 

 Nautilus. London, 1832.— The same, Article on Cephalopoda in Todd's Cyclopaedia. I. 1836. — 

 Vxlencibnnes, Nouvelles recherches snr le Nautile flambe\ Archives du Museum. 1841.— 

 Peteks, Anatomie der Sepiola. Arch. f. Anat. 1842. — Kolliker, Entwickelungsgesch. der 

 Cephalopoden. Zurich, 1814. — Van der Hoeven, BijcLragen tot de Ontleedkundige Kennis aan- 

 gaande Nautilus pompilius. Amsterdam, 1856.— Grenacheh, Zur Entwick. d. Cephalopod. 

 Zeitschrift f. wiss. Zoolog. Bd. XXIV. p. 419.— Fol, H., Note s. 1. tlt?veloppement des Mollusqnes 

 pteropodes et cephalopodes. Arch, de zool. T. III.— Bobretsky, Untersuch. iiber die Entvrick- 

 elung der Cephalopoden. Nacbr. d. k. Gesellsch. der Freunde d. Naturkenntniss etc. zu Moskau. 

 Bd. XXIV. (Russian.) [Rat Lankestee, Development of Cephalopoda, Quart. Journ, Mic. Sci. 

 1875.] 



Form of the Body. 



§ 248. 



The general form of the body of the Mollusca must be regarded 

 as one so much altered by the relative positions of many organs, 

 owing to the formation of shells, that it has only been possible to 

 recognise a ground-form, which shall represent the common origin, 

 by comparing the earlier larval stages with several mature forms. 

 The Placophora have a worm-like larva, and a similar kind of 

 external metamerism is indicated by the number of circlets of cilia 

 seen in the Gynmosomatous Pteropoda. The relations thus implied 

 are retained by the Placophora in their mature condition, at least in 

 the dorsal portion of the body. This is separated from the ventral 

 portion by a groove, and so defines two regions, which are found 

 also, under the form of " mantle" and "foot," although much 

 changed, in the Conchifera. The differentiation of a gutter-like 

 ventral surface in the Solenogastres (cf. p. 130), as has been already 

 explained, points to the Mollusca having genetic relations to these 

 worms ; this supposition is confirmed by the characters of the 

 nervous system. 



The Lamellibranchiata and Gastropoda, as well as the thecosomn- 

 tous Pteropoda, develop a well-marked circlet of cilia in the region, 

 which, later on, corresponds to the head ; this circlet is afterwards 

 carried on a special, symmetrical, and lobed process — the Velum. 

 The primitive significance of this circlet is clearly shown by its 

 presence in otherwise divergent divisions, and is even still more 

 important from the fact that we can recognise in this organ the 

 circlet of cilia which surrounds the same part of the body in many 

 Vermes (cf. § 107). The velum of the Mollusca may therefore be 

 regarded as an organ inherited from a lower stage. 



Below the velum the rudiment of the opening into the enteric 

 cavity is formed. As in the Placophora, the formation of a dorsal 

 shell in the Lamellibranchiata does not prevent the enteric tube 

 from being continued to the aboral pole of the body ; for in the 

 Placophora the shell, as well as the mantle which carries it, is adapted 

 to the whole body, and in the latter it is principally developed at the 

 sides. We are able, therefore, to distinguish a primary axis, which 

 extends from the oral to the anal pole, and which is crossed by two 



