BODY-CAVITY 399 



ditions which have been laid down about the characters of a true 

 coelom. As to the complexity of this system, attention may be 

 directed to the accompanying diagram (Fig. 206) of the coelom of 

 a segment of Glossiplionia, which has been lately worked out in 

 detail. 1 It will be observed that there are four main longitudinal 

 sinuses which are connected by a complicated system of transverse 

 tubes and spaces. In the anterior part of the body, before the 

 point where the intestinal caeca arise, the dorsal and ventral 

 lacunae fuse to form one larger so-called median lacuna. The cavity 

 of this is interrupted, in correspondence with the segmentation of 

 the body, by septa exactly comparable to those of Oligochaeta ; 

 but the septa in Glossiphonia are not present at every segment. 

 So far our account of the coelom is chiefly derived from the genus 

 Glossiphonia. In Hirudo, which is an example of the Gnathob- 

 dellae, the coelom is still further reduced ; the lateral sinuses in 

 them appear to be absent. But on the other hand there is formed 

 a series of cavities in a form of connective tissue which has been 

 termed botryoidal tissue. The cells of this tissue become hollowed 

 out, and form channels which are in communication on the one 

 hand with the remains of the coelom and on the other with the 

 vascular system. This system has certain analogies with the 

 lymphatic vessels of Vertebrates, and, like them, is an intermediary 

 between the body-cavity and the blood. Originally, however, 

 these botryoidal vessels have nothing whatever to do with either 

 the vascular or the coelomic system ; their connexion with both 

 is a purely secondary affair, and only appears, comparatively 

 speaking, late in life. 



The development of the spaces here spoken of collectively as 

 coelom confirms this interpretation of their nature. In the 

 embryos of Hirudo, Aulastomum, and Nephelis there is a ventral 

 space, 2 which includes the nerve-cord. Into this open a series of 

 paired and segmentally-dis posed lateral cavities, a pair to each 

 segment. The ventral cavity itself is formed by fusion of the 

 median parts of the lateral cavities. There is here clear evidence 

 of a coelom, developed on a plan fundamentally identical with 

 that of the Oligochaeta in that it is formed as a paired series 

 of chambers corresponding to the segmentation of the body. 



Nephridia. The " segmental organs " or nephridia are seen in 



1 Asajiro Oka, Zcitschr. iciss. Zool. Iviii. 1894, p. 79. 

 2 See Burger, quoted on p. 403. 



