296 THE INVERTEBRATA 



arranged in a single bundle on each side of each segment ; with separate 

 sexes, each with complicated genital apparatus, the females with 

 spermathecae and males with a pair of protrusible penes in each 

 segment behind the oesophagus. 



Nerilla (Fig. 204 A). As in Protodrilus, but with two bundles of 

 chaetae separated by a single cirrus on each side of each segment; 

 three prostomial tentacles and a pair of palps ; with separate sexes and 

 a reduced number of genital segments (three in male, one in female), 

 three pairs of sperm ducts uniting at a common median genital 

 aperture, and two oviducts with separate genital apertures. 



Dinophilus (Fig. 204 D) with very short flattened body consisting 

 of only five or six segments, a ciliated ventral surface and ciliated ring 

 in every segment; without septa, dorsal and ventral mesenteries, and 

 a vascular system; with greatly reduced coelom and longitudinal 

 muscles; five pairs of *' closed" nephridia; separate sexes, male with 

 median penis injecting spermatozoa into female through skin, female 

 with eggs of two sizes, the smaller giving rise to males and the larger 

 to females. 



Histriobdella^ which may be mentioned here (Fig. 204 C), is a 

 parasite of the eggs of the lobster, having no chaetae but two pairs 

 of "feet" by which it executes acrobatic movements. It resembles 

 Dinophilus in its reduced coelom and musculature but has jaws, and 

 from the structure of these it has been claimed that Histriobdella is a 

 much modified polychaet belonging to the family Eunicidae. 



The value of the Archiannelida to the elementary student of 

 zoology is that they illustrate an evolutionary process which may be 

 called simplification or reduction (but not degeneration), and which 

 is not unlike the changes which parasitic forms have undergone. 



Class HIRUDINEA 



Annelida with a somewhat shortened body and small, fixed number 

 of segments, broken up into annuli and without chaetae (except in 

 Acanthohdella) or parapodia ; at the anterior and posterior ends several 

 segments modified to form suckers; coelom very much encroached 

 upon by the growth of mesenchymatous tissue and usually reduced to 

 several longitudinal tubular spaces (sinuses) with transverse com- 

 munications. Hermaphrodite, with clitellum. Embryo develops in- 

 side cocoon. 



In the typical leeches the constitution of the body is remarkably 

 constant. There is a prostomium and thirty-two body segments; an 

 anterior sucker (in the centre of which is the mouth) is formed from 

 the prostomium and the first two segments, and a posterior from the 

 last seven. Both suckers are directed ventrally. The subpharyngeal 



