OF CONCHOLOGY. 91 



more or less bent upon itself, and in most cases with the excur- 

 rent orifice adjacent to the incurrent or oral opening. 



The muscles of Salpa abound in transversely striated fibres 

 ^V^hile among the mollusca transverse bands of muscular fibre 

 are not uncommon, yet these never enclose a succession of simi- 

 lar nervous, vascular or reproductive organs ; nor do they in- 

 clude the whole body except in some Tunicates. The majority 

 of the molkisca possess a calcareous shell, secreted by an organ 

 called the mantle, to which they are permanently attached by 

 strong muscles, and from which they cannot emerge and live. 

 Our knowledge of their embryology is too limited to admit of 

 sweeping generalizations. 



Chitine is very generally secreted by them in the form of 

 teeth, jaws, byssus, bristles* or opercula. 



On the part of the Articulates, and especially the Annelids, 

 their most strongly marked and typical feature is that of a 

 repetition of similar parts ; " we find it equally conspicuous 

 among the internal organs, the intestines, heart, lungs or gills, 

 nervous system and the reproductive system," " the highest of 

 them do not attain to that singleness of character which exists 

 even in the lower middle ranks of mollusca. "f 



Their principal structural characters are as follows : 



The circulator^/ system is highly developed, complete and 

 closed. The place of a heart is supplied by several large con- 

 tractile vessels,! or by the contraction of the vascular channels 

 or of the body itself.§ The vessels anastomose at both extremi- 

 ties, and by transverse vessels in the separate segments. The 

 blood is colored or colorless. 



The nervous system is composed of two cords Avhich connect 

 successive ganglia, or, instead, send off lateral branches at 

 regular intervals, along the median line of the body, while the 

 most anterior and principal ganglion lies above the oesophagus. || 



The digestive apparatus almost without exception opens by a 

 mouth at the anterior end of the body and terminates by an 

 anus at the posterior end. It is usually straight, rarely having 

 convolutions and frequently divided by muscular constrictions 

 into many sections. 



The muscles of the Annelids, though highly developed, appear 



* The bristles of Chiton are well known, in Acanthochites especially 

 they are gathered in regular bundles, in follicles, corresponding to the 

 segments of the shell, while in Cryptochiton they are spread over the en- 

 tire surface. 



t Mind in Nature, p. 214. 



j Siebold. Anat. Inv., p. 167, et seq. Burnett's translation. 



5 In the Nemertoida. 



II Siebold, p. 156-60. 



