EXCRETORY ORGANS. 27 



be obtained until the development is studied, but the main 

 features in its tojoographical arrangement seem fairly clear, and 

 may be described at this point. In the adult there are many 

 spaces in the tissues the exact nature and origin of which is not 

 understood. Such will be referred to by the general term lymph 

 spaces, f It has been stated by some observers (Scluieider, 

 Lankester and others) that the coelom and vascular system are 

 in certain parts of the body continuous. But having regard to 

 the doubtful character of many of the body-spaces above i-eferred 

 to and to the difficulties to investigation presented by the 

 vascular system, this statement cannot be accepted without 

 further evidence. 



In the region of the intestine there is a perivisceral cavity which 

 is coelomic. It entirely surrounds the intestine except dorsally, 

 where it is interrupted by the mesentery. In the region of the 

 pharymx the same cavity is found, but it is broken up by the 

 gill-slits into a number of parts all continuous with each other. 

 There are two dorsolateral chambers, one on each side of the 

 hyperpharyngeal groove. These extend a little way outward 

 in the lateral walls of the atrium, and dip down into the folds 

 of its roof along the primary bars. They constitute the dorso- 

 pharyngeal coelom, and are continuous through the above- 

 mentioned folds with the coelom present on the outer side of 

 each primary bar. Ventrally the coelom of the primary bars 

 opens into a median ventral chamber below the endostyle, 

 called the endostylar coelom. 



The arrangement of the coelom about the mouth, which has been 

 described by v. Wijhe, is too complicated for description in this work : 

 it is, however, referred to in the section on development. 



Excretory organs. In the dorsal wall of the atrium lying between the 

 atrial epithelium and the epithelium of the dorso-pharyngeal coelom are 

 a number of tubes with a ciliated lining, which have been supposed to be 

 renal in function (Figs. 12, 13). They correspond in number and position with 

 the primary gill-slits and do not extend behind the region of the pharynx. 

 They open into the atrium opposite the dorsal end of a tongue bar and at 

 the summit of one of the dorsal pouches of the atrium found at that point 

 (p. 26). They divide into two canals close to the opening ; one of these 

 passes forward and then turns round to travel for a short distance ventral- 

 wards ; the other passes backwards. They possess a variable number of 

 branches (from 1-5), the number being least at the two ends of the series 

 and greatest in the middle of it. Curious fibres ending in small knobs, 

 each of which contains a nucleus, pass off from the ends of these branches 

 and project into the coelom. These fibres are fine tubes ending blindly 

 internally and opening into the secretory tube. They contain a long 



