166 COMPAEATIVE ANATOMY. 



well developed in the Eotatoria and in most Annulata. It forms a 

 continuous^ and generally a very widoj cavity in tlie Bryozoa and in 

 the Gephyrea. The coelom of the Annulata is arranged in correla- 

 tion with the metamerism of the body. Partitions (dissepiments) 

 extend from the wall of the body to the enteric tube, and form 

 a series of separate chambers, each of which corresponds to one 

 segment of the gut, etc. ; these characters have been already 

 pointed out in the Nemertina. When the dissepiments are 

 reduced to single chords the chambers are more or less com- 

 pletely fused. In many cases this causes the separate chambers 

 to disappear, either along certain tracts, which are generally placed 

 in the anterior region, or along the whole length of the body ; and 

 a single body-space is formed, which is, generally, still traversed by 

 remnants of the dissepiments, in the form of filaments or fibrous 

 bands. These fibres keep the enteron in its place, especially when 

 it is coiled (cf. Bonellia, Fig. 72, m). 



The perienteric fluid is generally quite clear, and in most 

 Vermes contains form-elements, of which there are sometimes a 

 large quantity. When there is communication between the vascular 

 system and the coelom, the contents of the two spaces are similar in 

 character. The movement of the fluid is dependent on the action 

 of the body-wall, so that in many, locomotion of the body produces 

 at the same time a circulation of the nutrient fluid : and thus is 

 established the lowest form of circulation. 



The coelom communicates with the surrounding medium, the 

 water, by means of various arrangements. The excretory apparatus, 

 with its internal orifices (cf. § l-i2), is one, but special openings are 

 also known. For example, there is an opening of this kind in the 

 Bryozoa, which serves also as the passage for the generative pro- 

 ducts, and in the Rotatoria, where the orifice is generally drawn ont 

 into a tube (siphon : cf. Fig. 81, s). Similar orifices have been 

 observed to be present in the Annelides. 



Vascular System. 



§137. 



The differentiation of hollow spaces in the mesoderm is the first 

 beginning of the formation of a complicated system of canals ; which 

 gradually acquire special walls and become blood-vessels. The 

 earliest main trunks form long canals, which are first visible in the 

 Nemertina. Two (Fig. 76, II) of the three chief trunks take a lateral 

 course ; the third is dorsal and median. In the ceiDhalic region the 

 lateral vessel forms several coils, which surround the cerebrum, and 

 are connected with the dorsal vessel as well as with one another 

 more anteriorly. At the posterior end of the body all three vessels 

 are more simply connected. In some genera other vessels are con- 

 nected with these three ; five transverse vessels connect the dorsal 



