210 INVERTEBRATE MORPHOLOGY. 



from which numerous papillse or in some cases branching 

 processes arise, projecting into corresponding cavities or 

 tubes in the substance of the sholl-valves. Below the ecto- 

 derm is a more or less homogeneous connective tissue con- 

 taining cells and recalling the mesogkea tissue of the Coelen- 

 terates. Scattered muscle-fibres, arranged transversely and 

 longitudinally, occur in the mantle-lobes and in the body-wall, 

 but there are no definite muscular layers such as are found in 

 the Annelida, though the longitudinal muscles of the peduncle 

 are well developed. Special muscles, which cannot be con- 

 sidered differentiations of the musculature of the body-wall, 

 traverse the ccelom from one valve of the shell to the other, 

 one pair, the divaricators, being inserted in such a way as to 

 cause by their contraction a separation of the two valves, 

 while another pair, the adductors (Fig. 120, am), approximate 

 them. Other muscles also occur, such as the adjustores, 

 which produce lateral movements of the shell-valves, and pro- 

 tractors and retractors (Fig. 120, rm) of the peduncle. 



The coelom is lined by a peritoneal epithelium and con- 

 tains a corpusculated hsemolymph which is driven about 

 through the coelomic spaces, and the lacmife in the mantle- 

 folds and the lophophore which communicate w r ith them, by 

 the contractions of the body-wall and the musculature, there 

 being no distinct heart or blood-vessels. A dorso-ventral 

 mesentery which slings the intestine is present and divides 

 the body-coelom more or less completely into two lateral 

 chambers, and furthermore two transverse partitions or dis- 

 sepiments occur in several forms and divide the coelom into 

 anterior, middle, and posterior compartments, an arrangement 

 recalling the metamerism of such a form as Sagitta (p. 187). 



The mouth opens at the anterior end of the body between 

 the two lophophoric arms and leads into a short, somewhat 

 muscular oesophagus, which posteriorly communicates with a 

 stomach-like dilatation (Fig. 120, i) into which open the 

 ducts of one or more pairs of branching tubular glands the 

 so-called liver or digestive glands (I). Behind the stomach lies 

 the intestine, which, in most of the Ecardines, such as Lingula, 

 bends upon itself and opens into the mantle-cavity in the 

 mid dorsal line near the anterior end of the body. In Crania, 



