LESS, xxvii MUSCULAR SYSTEM 329 



The body is divided into a head (//</) and thorax (77/), together 

 constituting the cephalothorax (C. 77/), and seven free abdominal 

 segments {Abd. seg. i, Ahd. seg. 7) : the head is produced in front into 

 a rostrum (A'). 



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

 exoskeleton, deric epithelium {Der. Epthtn), 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 

 (St), from which a short small intestine (S. hit) leads into a large in- 

 testine (/.. hit), 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 

 mesodermal (connective tissue and muscle). 



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

 a blood-sinus. 



The heart (///) 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. Gi/i) are enclosed in a cavity formed by a fold of the 

 thoracic body-wall called the branchiostegite (^^'j/^"^) : 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 (Gii) united by connectives. 



The ovary ((rvy) is a hollow organ opening by an oviduct (b, ovd) on 

 the base of one of the legs (Leg). 



muscle {env. m) in the form of a loop, the outer limb of 

 which {fl) turns forwards and is inserted into a sternum, 

 while the inner limb {fl) turns backwards and is inserted 

 into another and more posterior sternum. The contraction 

 of this enveloping muscle produces an approximation of the 

 sterna, and thus flexes the abdomen, the central muscle 

 always keeping the middle of the loop in place. The 

 ventral muscles are, like the dorsal, traceable into the 

 thorax, where they arise from the endophragmal system : 

 their various parts are connected by a complex system of 

 fibres extending between the central and enveloping muscles, 

 and connecting both wiih their fellows of the opposite side. 

 The flexor muscles are immensely powerful, and produce, 

 when acting together, a sudden and violent bending of the 



