MUSCLES. CCELOM. NERVOUS SYSTEM. 437 



lying on the haemal side of the cephalic caecum, with two slender arms extending 

 backward on either side of the pharynx. (Fig. 297, en.c.) The body of the en- 

 docranium consists of a sharply defined mass, consisting of clear, concentric 

 "chitenoid" lamellae, said to arise from the basement membrane of the cephalic 

 caecum and the pharynx, and in some cases containing cells lodged between the 

 lamellae. The lateral portions are less clearly defined and consist of a cartilage- 

 like matrix, containing small clusters, or nests of cartilage cells, said to be derived 

 from the epidermis. 



Whatever may be the origin of the endocranium in the enteropneusta, it 

 certainly bears a strong resemblance in its location, form, and histological struc- 

 ture to the endocranium of the arachnids and phyllopods. 



Muscles. — The arrangement of the muscles in the collar and trunk supports 

 the interpretation indicated above. We have shown (p. 230) that in the arachnids 

 practically all the somatic, or intersegmental, muscles are absent in the thorax, 

 this condition being either the cause or the result of the fusion of all the thoracic 

 and cephalic tergites into one cephalothoracic shield. The muscles that remain 

 belong to the appendages, or to the forward extension of longitudinal abdom- 

 inal muscles that are attached to the posterior portion of the endocranium. There 

 is, therefore, a marked difference in the arachnids as well as in many other arthro- 

 pods between the musculature of the thorax and that of the abdomen. According 

 to Ritter there is in Balanoglossus a similar difference between the musculature 

 of the collar (thorax) and trunk. He states that circular somatic muscles are 

 wholly wanting in the collar. Here the principal muscles are radio-longitudinal, 

 attached to the posterior wall of the collar at one end, and at the other mainly to 

 the "notochord" and nuchal skeleton, but also to the walls of the oesophagus. 

 The muscles of the branchial and caudal regions, both longitudinal and circular, 

 are always strictly somatic. 



The Coslom consists of three main divisions. (Figs. 279, 298, c.^'^.) That 

 in the proboscis, c', may be regarded as the remnants of the procephalic coelom, 

 and is drained by one or two excretory ducts, comparable with the antennary 

 ducts of Crustacea, or the cheliceral ducts of the arachnids (Galeodes). The 

 collar coelom, c^ represents that of the thorax, and its excretory duct represents 

 one of the posterior thoracic ducts, i.e., the so-called shell gland of the phyllopods 

 or the coxal gland of the arachnids. The trunk coelom represents that of the 

 abdominal and post abdominal segments united to form a continuous chamber, 

 and is devoid of excretory ducts as it is in the arthropods generally. (Com- 

 pare with Fig. 279.) 



The Nervous System. The Neural and Hcemal Cords. — The nervous system 

 of arthropods forms : i. A "ventral" medullary plate consisting of parallel, gang- 

 lionated, segmented cords, perforated by the primitive stomodaeum. It coincides 

 with the primitive axis of growth and differentiation, ontogenetically and phy- 

 logenetically. It is the primary nervous system because it is the oldest, and 

 because it is always associated with the oldest sense organs, muscles and append- 



