GENERAL CHARACTERS 313 



The body is divided into a head (Hd] and thorax ( Th), together 

 constituting the cephalo-thorax (C. Th}, and seven free abdominal 

 segments (Abd. seg. I, Abd. seg. 7) : the head is produced in front into 

 a rostrum. 



The body- wall consists of cuticle (Cu), partly calcified to form the 

 exoskeleton, deric epithelium (Der. Epthni), dermis (Derm.}, and a 

 very thick layer of muscle (M) which in the abdomen is distinctly 

 segmented. 



The mouth (Mth} leads by a short gullet (Gul} into a large stomach 

 (6Y), from which a short small intestine (S. Ini} leads into a large in- 

 testine (L. Itit}, ending in the anus (An}. Opening into the small 

 intestine are the digestive glands (D. Gl}. The epithelium of the small 

 intestine and digestive glands is endodermal, that of the rest of the canal 

 is ectodermal and secretes a cuticle : the outer layer throughout is meso- 

 dermal (connective tissue and muscle). 



The cavity (B. S) between the enteric canal and the body-muscles is 

 a blood- sinus. 



The heart (Hi} is enclosed in the pericardial sinus (Per. S) : the 

 chief ventral blood-vessel or sternal artery (St. A} is shown in B. 



The gills (B, Gill} are enclosed in a cavity formed by a fold of the 

 thoracic body-wall called the branchiostigite (Brstg] : they are formed 

 of the same layers as the body-wall, of which they are offshoots. 



The kidneys (A, K] are situated in the head. 



The brain (Br) lies in the prostomium : the ventral nerve-cord ( V. 

 Nv. Cd} consists of a chain of ganglia (Gn) united by connectives. 



The ovary (ovy] is a hollow organ opening by an oviduct (B, ovd} on 

 the base of one of the legs (Leg). 



stomach (67), and a straight intestine divisible into a short 

 anterior division or small intestine (S. Int.] and a long 

 posterior division or larger intestine (L. Int.] : the latter 

 opens by an anus (An) on the ventral surface of the last 

 segment. The study of development shows that the only 

 part of the canal derived from the enteron of the embryo is 

 the small intestine : the gullet and stomach arise from the 

 stomodseum, the large intestine from the proctodaeum. 

 Thus the only portion of the enteric epithelium which 

 is endodermal is that of the small intestine : the epithelium 

 of gullet, stomach, and large intestine is ectodermal, and, 

 like the deric epithelium secretes a cuticle. The outer 

 layer of the whole enteric canal consists of connective 

 tissue and muscle : there is no coelomic epithelium. 



