T7PE ANNELIDA. 231 



Nemerteans (p. 165), the lowest forms that possess a distinct blood vascular 

 system. In this group the coelom, so far as it exists, consists of small spaces 

 without any definite walls scattered through the parenchyma. In some 

 forms the blood vascular system communicates with these spaces through 

 ■which the blood circulates, it being only in the most highly differentiated 

 Nemerteans that the vascular system is closed. It might be supposed from 

 this that the blood-vessels were simply coelomic spaces which had acquired 

 definite walls; and it seems probable that such has been their origin. In the 

 Annelida a somewhat different state of affairs occurs. Here, as a rule, there 

 is a definite coelom lined with peritoneum and completely separated from the 

 cavity of the blood-vessels, which seem to represent rather the remains 

 of an original cavity, the so-called blastocoel (see p. 52), which has been 

 almost obliterated by the growth of the mesodermal segments by the hollow- 

 ing out of which the coelomic cavities have been formed (see p. 56). It 

 seems certain that the coelomic spaces of the Nemerteans are likewise the 

 remains of a primitive blastocoel, so that to this extent the homology of the 

 blood-vessels holds in the two groups. 



In the Hirudinea, however, the blood-sinuses, if they are coelomic, cor- 

 respond with the coelom of the Polycheeta ; and furthermore, in the Oligo- 

 chsets and Polychsets, as well as in the Gephyrea, as will be seen later, the 

 hsemolymph contained in the coelom is very nearly if not quite identical in 

 composition with the blood contained in the blood-vessels. These facts 

 would seem to indicate a close relationship between the Annelid coelom and 

 the more primitive blastocoel; or, in other words, would lead us to suppose 

 that the coelom of the Annelids lined with peritoneum is not something 

 apart and distinct from the blastocoel cavity, as has usually been supposed. 

 The view which maintains the distinctness of the two forms of coelom has 

 its origin in the fact that in some forms, such as Sagitta, a coelom lined with 

 peritoneum is formed as an outgrowth from the primitive digestive tract; 

 and it was supposed that all coelomic cavities with definite walls were pri- 

 marily of a similar origin, and hence were termed enterooals in contradis- 

 tinction to the schizoccels or simple spaces in the mesoderm without defi- 

 nite walls, which are in reality remnants of the blastocoel. The significance 

 of true enteroccels will be discussed later. In the mean time it may be 

 pointed out that there is no embryological evidence in favor of the Annelid 

 coelom having arisen as a series of pouch-like outgrowths from the primi- 

 tive digestive tract. It is rather to be regarded as a sohizocoel whose char- 

 acter has been altered by metamerization, and by the manner of its forma- 

 tion from mesoblasts. On this view the union of the cavity of the blood- 

 vessels with the coelom in the leeches, and the similarity of the hsemolymph 

 to the blood in other forms, cease to be morphological puzzles. 



The mouth lies at the bottom of the anterior sucker and 

 opens into a muscular pharynx, which in some forms (e.g. Clep- 

 sine) is folded similarly to that of some Turbellaria (see p. 134) 

 so as to form a protrusible tube, while in others (e.g. Hirvdo, 



