THE ANATOMY. CLITELLUM 141 



(1) In most of the aquatic families, in the Perichaetidae, many Cryptodrilidae, 

 and Acanthodrilidae, in the Eudrilidae, one or two Geoscolicidao, and in the Monili- 

 gastridae, the clitellum extends right round the body ; that is to say, the peculiar 

 modification of the epidermis is apparent throughout the entire circumference. 

 A clitellum of this kind is sometimes spoken of as being 'complete.' ROSA has 

 suggested that the term cingulum be applied to this form of clitellum, expressing 

 its girdle-like character, and that the term clitellum should be restricted to the 

 variety of the organ next to be described. 



(2) In all Lumbricidae, nearly all Geoscolicidae, and in some worms belonging to 

 the Megascolicidae, the clitellum has a saddle-shaped form ; this is due to the fact 

 that the epidermis of the ventral surface, to a variable extent (variable according 

 to the species or genus), has not been invaded by the glandular modification of the 

 epidermis. The clitellum has therefore a saddle-like shape l . ROSA attempted, at one 

 time, to draw a hard and fast line between this kind of clitellum and the last. It 

 is, however, not possible to draw such a line in a satisfactory way. The extremes, 

 it is true, differ ; the clitellum of Lumbricus is very different from the cingulum 

 of Perichaeta ; but there are intermediate conditions. In some worms which appear 

 to have a completely encircling clitellum, the glandular layer is distinctly thinner 

 on the ventral surface ; in several species of Acanthodrilus the clitellum is complete 

 anteriorly and incomplete posteriorly. In Pontoscolex coretkeura the first one or 

 two segments of the clitellum are complete ; then follow a few in which the median 

 area free from glandular modification is narrow ; in the succeeding segments again 

 this area becomes wider. The same differentiation of width in the ventral area 

 occurs in Rhinodrilus gulielmi. 



(3) Aeolosoma has a form of clitellum which is peculiar to that genus. The 

 peculiarity consists in the fact that it is developed only on the ventral side of the body ; 

 this form of clitellum is therefore the exact converse of the saddle-shaped clitellum 

 just described. 



There is a great variation, among the Oligochaeta, in the segments which are 

 occupied by the clitellum. This valuation affects not only the number of segments 

 included but their position ; and most useful specific, generic, or even family 

 characters are obtainable from these variations. As a general rule, there is some 

 relation between the position of the clitellum, and that of the other organs of reproduc- 

 tion. Among the aquatic Oligochaeta there is a distinct relation between the position 

 of the male-pore and that of the clitellum. In the Naidomorpha the clitellum is 

 quite anterior in position, so too are the orifices of the sperm-ducts, which open 



1 'Similjininin opliippio quod anterior! arcu caret' (WILLIS). 



