92 AMERICAN JOURNAL 



without notable exception to be destitute of transverse stria- 

 tions.* 



The whole body is enveloped by subcutaneous muscular layers ; 

 (1) of transverse muscles ; (2) of longitudinal muscles, and in 

 many forms a third, of obliquely twisted fibres, is interposed 

 between the two. 



Such of the Annelids as possess a shelly covering form it by 

 a secretion from a collar which surrounds the anterior extremity, 

 which either deposits a calcareous matter or a fluid which serves 

 to agglutinate grains of sand or other substances together to 

 form a tube. But in no case are the inhabitants provided with 

 permanent muscular attachments to such tubes ; they can emerge 

 without injury, especially when young,f and form another habi- 

 tation without inconvenience. The shells of Annelids are al- 

 ways univalve, and no Annelid is known to attach itself by a 

 byssus or by a portion of its integument, to any extraneous 

 object. 



In the Brachiopods and Polyzoa we find the following details 

 of structure : while it is noteAvorthy that they are formed on the 

 plan of uniformity, characteristic of the mollusca, and exhibit 

 no trace of the reduplication of similar parts, which is the most 

 salient feature of the Annelids and articulates in general. 

 There is no vascular system or heart in the Polyzoa. The heart 

 in the Bracldopoda is indistinct and the circulation is incom- 

 plete, recalling a similar condition of things among the ascidians. 

 Two or four accessory, contractile vesicles assist in propelling 

 the blood. They are distinct from the blood vessels, in which 

 the circulation is assisted by ciliary action. The blood is colored 

 as in Chiton and many Ascidians. The nervous system in the 

 Polyzoa consists principally of a ganglion placed between the 

 afl"erent and efferent openings as in Ascidians. In the Brach- 

 iopods it is more highly developed and consists of a series of 

 ganglia and nerves, forming a collar surrounding the oesophagus 

 " as in all true mollusks," with the greater portion of the gan- 

 glia below the oesophagus, while in worms it is above it. 



The digestive canal is bent upon itself in both Polyzoa and 

 BracJtiopoda, without exception. This is doubtfully reported, 

 as far as I know, of but one worm, a larval form [Phoronis) ; 

 other characters of this Annelid will presently be mentioned. 

 In some of the Brachiopods the canal is closed, while in others 

 it opens laterally. It is never divided into sections by muscular 

 constrictions as in worms. 



*Siebold, 1. c. p. 154. But one or two species of Aunelids have been 

 asserted to have striated muscular fibres. 



t Johnstone, Cat. Brit. Nouparasitical worms, pp. 1 k 81. 



