OF ARTS AND SCIENCES. 77 



It is a well-known fact that the end somites are incomplete in re- 

 spect to the number of rings composing them ; but the mode and 

 extent of abbreviation have not hitherto been made the subject of 

 careful study ; and hence some of the more important generic charac- 

 ters have been entirely overlooked, and with them some points of the 

 highest morphological interest. Hirudo is the best known and most 

 widely distributed genus, and on this account forms a convenient 

 standard of comparison. As the arrangement of the segmental sense- 

 organs enables us to determine the exact number of somites composing 

 the body, we may deal with this part of our subject first. 



The Segmental Sense-organs of the Leech. — The only pense-oi'gans 

 hitherto known in the Medicinal Leech are the five pairs of eyes and 

 the so-called "goblet-shaped" organs located on the lip (cephalic 

 lobe). A number of writers have noticed and described some small 

 spots, occurring on every fifth ring of the body ; and one author has 

 suggested that they may have a respiratory function. These spots 

 when examined closely with a low magnifying power will be found to 

 be slight elevations with rounded summits ; and for this reason, and 

 because they are regularly disposed on the first ring of each somite, 

 they may be called segmental papillcB, a name which does not prejudge 

 the question of their function. 



In our large Pond Leech (Macrobdella Verrill), these papillae are 

 comparatively small ; and the same may be said of the medicinal 

 leeches of Europe and Japan, and their nearest allies, Ha^mopis and 

 Aulostoma. In some of the Asiatic medicinal leeches, for example, 

 those of Saigon, Singapore (II. maculosa) , Java (II. javanica), and 

 Ceylon (H. mnlti striata), they are much larger, and have an oval 

 form, with a median ridge or crest. In the land leeches they are very 

 conspicuous, having the form of small cones with rounded suiumits. 



In all the ten-eyed leeches of Japan, including both the land and 

 fresh-water forms, twelve of these papillae are found on the first ring 

 of each complete somite, six on the dorsal and six on the ventral side. 

 In most of the medicinal leeches, however, as well as in Haemopis, 

 Aulosloma, Macrobdella, etc., there are eight on the dorsal side, and 

 six on the ventral. 



A careful study of the arrangement of these papillae in a large num- 

 ber of species, and of their histological structure, has brought out in 

 a most conclusive manner their serial homology with the eyes ; and 

 has led, indirectly, to the recognition of some important points in re- 

 gard to the metameric composition of the body of the leech. The 

 arrangement of the papilla) on the dorsal side is shown in Fig. 1. 



