NO. 4 HARTMAN : POLYCHAETOUS ANNELIDS 393 



some species nephridia may be present in young individuals and later 

 undergo retrogression to disappearance in certain segments. Furthermore, 

 they may be present on some abdominal segments or some thoracic seg- 

 ments, or some segments in both thorax and abdomen. When immature 

 or only partly developed, they are sometimes distinguished with difficulty. 

 On the other hand, in sexually mature individuals, where they come to be 

 modifieH as genital pores, they may be very conspicuous (pi. 50, fig. 5). 

 On the thorax the nephridial pore is often in the segmental groove at the 

 sides of the body (pi. 47, fig. 1) ; on the abdomen it takes various positions 

 depending on the species, but it is usually in the vicinity of parapodia or 

 lateral organs, when present. In some species (Capitella) nephridia are 

 polymeric on some abdominal segments ; the apertures are in the epithe- 

 lium and not easily visible in external view (see Eisig, 1887, p. 272). 



A closed circulatory system is absent in the capitellids. The ventral 

 chamber of the coelom substitutes as a haemocoel. The various parts ot 

 the coelom (visceral, branchial, and gonadial chambers) are in com- 

 munication with one another through definite openings. The circulatory 

 fluid moves in the coelom with the lymph and is freely mixed with the 

 gonadial products, but rhythmic contraction insures that the blood cir- 

 culates in the branchiae, prostomium, and other terminal parts (see also 

 Haffner, 1930, p. 136). The circulatory fluid is red owing to the pres- 

 ence of corpuscles. 



Modifications of body parts are less marked in the capitellids than in 

 most chaetopods. The prostomi\im, peristomium, and thorax are typically 

 plain. Parapodial ridges or parts of the surface epithelium may be modi- 

 fied as special structures to function for respiration and for glandular 

 secretion. In some species there are extensile, filamentous (pi. 51, fig. 4) 

 or tufted (pi. 50, fig. 6) prolongations functioning as branchiae; they 

 may be stationary (pi. 58, fig. 6) or retractile (pi. 55, fig. 4) and are 

 usually near the parapodial ridges on some abdominal segments. In some 

 others the pygidium may be modified as a caudal appendage (pi. 45, fig. 

 4) presumably for respiration, reminiscent of a similar structure in some 

 aquatic oligochaetes. 



The specific character of the hooded hooks or uncini is of particular 

 significance because of the repetition of these parts through most body 

 segments and their striking constancy throughout the segments of an 

 individual and among the individuals of a species. In all instances where 

 these details have herein been carefully worked out, it is found that these 

 hooks are not only highly specific (contrary to the opinions of some 



