THE OLIGOCH^TA. 193 



the mesoderm of the adult. And it is this segmentation of 

 the mesoblast, and consequently of the mesoderm, which con- 

 stitutes the most important difference between the Leech on 

 the one hand and the Turbellarian and Trematode on the 

 other. 



On the other hand, in the development of a mesoblast 

 which undergoes division into segments, the Leeches exhibit 

 the fundamental character of all such segmented Invertebrates 

 as the chtetophorous Annelida and the ArthropoiUi. 



The Oligoch^ta. — The earthworm {Lumhrlcus) and 

 fresh-water worms (JV^ctis, Tubifex, Chcetog aster) ^ which are 

 included under this name, are closely allied with the Leeches 

 in the essential points of their structure and development, 

 much as they differ from them in habit and appearance. 



They have elongated, rounded, segmented bodies, often 

 divided by many superficial transverse constrictions into 

 rings, which, as in the leeches, may be more numerous than 

 the proper somites. There are no limbs, but each segment 

 is usually provided with two or four sets of longer or shorter 

 chitinous set;^, which are developed and lodged in integument- 

 ary sacs. The outermost layer of the ectoderm is a non-cili- 

 ated chitinous cuticle. 



The mouth is situated close to the anterior end of the 

 body, but a " cephalic lobe " not unfrequently projects be- 

 yond it on the dorsal side. The anus is at the opposite ex- 

 tremity of the body, and the straight alimentary tract which 

 connects the two and is lined by the endoderm is usually 

 divided into a pharyngeal, oesophageal, and gastro-intestinal 

 portion, the latter often being produced laterally into short 

 caeca. The mesoderm presents well-developed transverse, 

 longitudinal, and dorso-ventral muscular fibres, as in the 

 Leeches. It is excavated by a spacious perivisceral cavity. 

 Which contains a colorless corpusculated fluid, and is divided 

 by thin but muscular mesenteries, which stretch from the in- 

 testine to the parietes, and thus break up the perivisceral 

 cavity into partially separate chambers. In addition, there 

 is a system of pseud-haemal vessels, like those of the Leeches, 

 provided with contractile walls, and containing a red non- 

 corpusculated fluid. No communication has been ascertained 

 to exist between these vessels and the perivisceral cavity ; 

 but there can be little doubt that, as in the case of the 

 Leeches, they must be regarded as a specially differentiated 

 part of the general system of the perivisceral cavity. 



