CHAETOPODA 263 



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. 193 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 inside 

 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 

 "ganglion" (Fig. 194 B) is composed of four single ganglia fused 

 together and the posterior "ganglion" of seven. Between them lie 

 twenty-one free ganglia, and the number of segments is estimated by 

 summation of all the ganglia. The number of annuli to a segment 

 varies in different forms. 



The alimentary canal is highly characteristic and consists of the 

 following parts, (i) A muscular pharynx with unicellular salivary 

 glands. In the Gnathobdellidae, which includes Hirudo, there are 

 three chitinous plates or jaws. In the Rhynchobdellidae (Fig. 195), 

 there is a protrusible ^ro^o^m surrounded by a.proboscis sheath. (2) A 

 short oesophagus follows, leading into (3) the mid gut (crop) which is 

 often provided with lateral coeca, varying in number, and is used for 

 storing up the blood or other juices of the host. This is kept from 

 coagulating by the ferment (anticoagulin) contained in the salivary 

 secretion (Hirudo). In the mid gut a very slow digestion takes place, 

 the blood appearing almost unchanged even after several months. 



