132 ZOOLOGY 



for example, the segmentally-arranged nerve ganglia, may fuse 

 together, and thus the segmentation becomes irregular. 



The Hirudinea are the first group in which segmentation 

 forms a distinctive feature. The integument is ringed, and in 

 the medicinal leech, Hirudo medieinalis, five annuli correspond 

 with a segment, in Pontdbdella four, and in Branchellion three. 

 The Hmits of each true segment are, however, marked out by 

 the arrangement of the colour bands. 



A cuticle corresponding with the mucous tubes of Nemer- 

 tiaes is formed from the secretion of unicellular glands ; it is 

 constantly being worn off and replaced. 



The body-cavity is much reduced by the great developement 

 of muscles and connective tissue, and in the medicinal leech 

 its chief remains form the dorsal and ventral sinuses, it is, 

 however, more conspicuous in the Rhynchobdellidae. 



The bodies of those forms, such as Clepsine and N&phelis, 

 where the muscles are strongly developed and the connective 

 tissue is sparse, are peculiarly firm and rigid, but forms like 

 Aulostoma, and to a less extent Hirudo, where the connective 

 tissue predominates, are extremely limp and flabby. The cells 

 of this connective tissue are embedded in a gelatinous matrix, 

 and they may assume the following characters : (i.) fat cells, or 

 cells crowded with fat globules, common in Clepsine; (ii.) 

 elongated branched cells crowded with globules which are not 

 fat, these pass into fibres at times ; (iii.) pigment cells ; (iv.) 

 vaso-fibrous and botryoidal cells, forming a tissue which 

 is composed of certain rounded cells crowded with brown pig- 

 ment and arranged in rows ; by a change in their interior, 

 channels arise which pierce the cells, and these ultimately open 

 on the one side into the closed system of blood-vessels, and on 

 the other into the coelomic sinuses. The botryoidal tissue 

 appears to pass into the vaso-fibrous tissue by the cells 

 dividing and becoming small, and the walls becoming very 

 thin, and the nuclei of the cells dropping out. Many of the 

 minute capillaries thus formed run between the columnar 

 epithelial cells of the ectoderm, and here the oxygenation of 

 the blood takes place. 



In Nemertines the vascular system is one with the 

 coelomic. The space which contained the corpusculated fluid 



