440 ALFRED GIBBS BOURNE. 



Histological Characters of Hirudinean Muscle. — 



The muscles consist of very long cells arranged either in 

 bundles or lying singly. 



These cells may be much branched, e.g. in the wall of the 

 alimentary canal. They consist of a cortical layer which 

 exhibits longitudinal fibrillation (shown in transverse sections), 

 and a medullary substance granular in nature containing a 

 large oval nucleus. 



Connective and Vasifactive Tissue. — The comparative 

 study of the histological characters of the connective and vasifac- 

 tive tissues of the Hirudinea has yielded many interesting results, 

 most of which accord with and elucidate the result obtained by 

 Professor Lankester. I hope to study upon a future occasion 

 the development of the various modifications which exist in 

 the adult, from the simple embryonic tissue. Such a study is 

 necessary to corroborate and amplify the views here put 

 forward which are based upon the phylogenetic relations which 

 exist in the different genera. But even treating the subject in 

 this manner it seems to me possible to trace all the various 

 modifications in their development from the ordinary connective- 

 tissue corpuscle. 



Previous observations upon the minute character of these 

 tissues in the group are, in addition to the researches of Pro- 

 fessor Lankester upon Hirudo above mentioned, to be found 

 in the works of Leydig, who accurately describes and figures 

 pigment cells from Piscicola and fat cells from Piscicola 

 and Clepsine, and of Vaillant, who figures some of the 

 elements from Pontobdella but seems to have confounded 

 to some extent the true connective-tissue elements with the 

 glands, epidermic in nature, which lie amongst them. Vail- 

 lanf's memoir does not in fact profess to be so much histo- 

 logical as anatomical in character. 



The Connective Substance in the Hirudinea. — This 

 occurs in all the genera, presenting similar charactersbut diSering 

 in the extent to which it is developed. The amount of this tissue 

 which is developed is in direct proportion to the " limpness " 

 of the leech. Clepsine among the Rhyncobdellidae and 



