950 SUMMARY OP CURRENT RESEARCHES RELATING TO 



Vermes, 

 a, Annelida. 



External Morphology of Hirudinea.* — Dr. S. Apathy has made a 

 close study of the external body-form of Leeches. The typical somites 

 are found m the mid-body, and consist of a number of rings, conBtant 

 ^vithin the limits of the genus, and having, cither in part or all, special 

 distinf^uishinf marks. Among the Rhynchobdcllida> Branchcllion and 

 Clepsinc have three rings, CalliobdcUa, Ichthjohdella, and Pontohdella 

 six Piscicula twelve ; the Gnathobdellidso have five ringn. All the rings 

 have tactile goblets, of which Piscicola has eighteen in a transverse row ; 

 there are also one internal and one external paramedian, one external 

 and one internal paramarginal, and one marginal goblet on the boundary 

 of the rinf's. These goblets contain a group of specific epithelial cells, 

 which always carry a tactile seta each. Further distinguishing marks 

 arc afforded by the plexiform superficial pigment which forms dark 

 transverse bands, and by the position of the ncphridiopores. The 

 special marks of the several rings are regularly repeated in each somite 

 in which the ring is present, and the absence of these marks indicates 

 the absence of certain rings. 



All the somites of the other parts of the body are only modifications 

 of the typical somites of the mid-body. These modifications are seen 

 in the more or less well-marked chai-acter of the rings, in the appearance 

 of certain superficial foldings of the skin, and in the smoothing out of 

 foldings which are found in the somites of the mid-body ; but the most 

 marked character is the shortening and reduction of the somite, due to 

 change in or loss of function. 



The first form of shortening is the simple reduction in length of the 

 somite, without any fusion of the ring. In the mid-body the somites 

 are all of much the same length, except where secondary extension of 

 certain parts of the enteron combined with a thickening of the body has 

 produced a certain increase in length of the somite. It is to be noted, 

 however, that, in all genera, there is a regular increase in length of tlio 

 somite from the clitellum to the end of the body, and from the hinder 

 boundary of the mid-body to the sucking disc. 



The second form of shortening is the fusion of certain rings, ordi- 

 narily belonging to the same third of a somite, with one another, so that 

 in some genera where the typical somite consists of six rings, there are 

 only three independent rings. It is rare for rings of different thirds to 

 become fused by the secondary adaptation of tegumcntary folds. When 

 a somite of the Ehynchobdellida) is reduced the reduction is first seen in 

 the hinder ring ; if the reduction goes further the second ring is affected, 

 and beyond this reduction never goes. 



This law of reduction is due to the relation of the general function 

 of the typical somite to the three thirds of the internal somite. As soon 

 as a change of function of a given part of the body causes certain organs 

 to become superfluous, that third of the internal somite with which that 

 function or gi'oup of functions was connected disappears also; the 

 remaining third or thirds arc developed at the cost of what has dis- 

 appeared. The hinder third contains no organs which are necessary for 

 absolute existence, and so it is the first to disappear ; on the other hand, 



• Mittbcil. Zool. Stat. Neapel, viii. (1888) pp. 153-232 (2 ph.). 



