44 8 THE ANIMAL KINGDOM. 



and Roule, ibid. ii. 1885. Salensky on Salpa (supra}; Seeliger on Clavellina lepadi- 

 formis, SB. Akad. Wien, Ixxxv. Abth. i. 1882. Segmentation and postembryonal 

 development, E. van Beneden et Julin, Archives de Biol. v. 1884 ; vi. 1886. 



For literature, see Herdman, Tunicata, Challenger Reports, vi. 1882; xiv. I886 1 . 



PHYLUM MOLLUSCA. 



Coelomate Metazoa, which are primitively bilaterally symmetrical: 

 with a soft integument, generally ciliated and richly supplied with glands : 

 with an integumental fold forming a mantle or pallium, and a ventral 

 muscular thickening of the body walls known as the foot. There is usually 

 an external shell borne upon and secreted by the mantle. Vascular processes 

 of the body wall form gills or ctenidia adapted to aquatic respiration. The 

 nervous system consists typically of a pair of cerebral ganglia united by 

 connectives to a pair of pedal and a pair of pleural ganglia, the latter con- 

 tinuous with a ganglionated visceral loop, of greater or less extent. There is 

 a pair of otolithic vesicles or otocysts ; a heart, and a partly lacunar blood- 

 system ; one or two renal sacs or nephridia. A large gland or liver generally 

 opens into the alimentary canal. There is a characteristic larval form or 

 Veliger. 



Bilateral symmetry is retained in Cephalopoda, Scaphopoda, Lamel- 

 libranchiata, and some few Gastropoda. But in the majority of the last- 

 named class there is a marked external and internal asymmetry, the latter 

 shown by the position of certain of the organs (heart, termination of the 

 intestine) and the loss of others (a nephridium^a gill); the former by 

 the disposition of the dorsal part of the body or visceral dome and the 

 mantle fold, as well as by the shape of the shell. In some cases, however, 

 a secondary external symmetry has become established as in some Opistho- 

 branchia and Pulmonata. The Pteropoda, though as a rule externally 

 symmetrical, are internally asymmetrical. The mantle fold is disposed 

 in a characteristic manner in the different classes. It is never absent in 

 the embryo, though sometimes lost in the adult of some Pteropoda and 

 Gastropoda. It is a continuous fold except in Lamellibranchiata, where 

 it is formed by independent right and left folds. The same is the case 

 in the Scaphopoda but here the mantle folds unite or concresce ventrally 

 as they do in some Lamellibranchiata. The space inclosed between the 

 mantle fold and the body is the mantle- or sub-pallial cavity. The foot 

 assumes various shapes equally characteristic of the various classes : it 

 is rarely completely aborted as in certain Gastropoda (Opisthobranchia 

 Haplomorphd) and a few Lamellibranchiata (Ostreidae). It extends 



1 An account by Herdman of Larvacea, Pyrosoma, and Thaliacea will appear in a forthcoming 

 volume of the Challenger Reports. 



