64 



OLIGOCHAETA 



benhami, but it is not trifid. At a varying distance from the anus the typhlosole 

 ceases; 1 have figured its abrupt ending, in the Acanthodrilid Odockaetus multi- 

 porus. After this point the term rectum may be applied to the gut. 



The large intestine is generally a straight tube running vrithout a bend from 

 commencement to termination ; in a few forms, however, it has a spiral arrangement ; 

 this is seen in Plagiochaeta, and, according to Horst, in Pontoscolex corethrurus. 

 In the course pf the tube it often exhibits a specialization into several regions; in 

 Megascolex coeruleus this is especially evident — perhaps to some extent on account 

 of the large size of the worm rendering the various regions more evident than 

 they would be in a small species. In the first six segments the intestine is deeply 

 pouched ; in the twenty-second segment the pouches became deeper still ; they extend 

 to about the seventy-sixth segment; in several Oligochaeta the intestine commences 

 in this way with a series of deepish pouches; for instance, in Urobenus brasiliensis 

 (Benham, 3), where, however, it only extends from segment xvi. to xxv. In 

 Megascolex coeruleus there are a series of glands appended to the intestine and lying 

 beyond the pouches ; these are the ' kidney-shaped glands ' of my description of 

 that Annelid ; they were also found by Bourne in his specimens ; the actual number 

 of pairs of these glands appears to vary as both Bourne and I give different 

 numbers. Their structure is quite simple ; they have the appearance of being formed 

 by a much folded membrane ; it is a matter of some interest from a classificatory 

 point of view that entirely similar glands occur in the genus Typhoeus belonging 

 to the family Cryptodrilidae. Otherwise they are unknown in the Oligochaeta. The 

 number of pairs is less in Typhoeus. Highly characteristic of the genus Perichaeta, 

 but also, strange to say, found in the apparently remotely allied Urobenus, are a pair 

 of caeca of the large intestine ; these occur nearly, if not absolutely, always in the 

 twenty-sixth segment ; they are directed forwards, and occupy two or three segments. 

 A few species of Perichaeta, for example P. sieboldi, have a mass of six or seven of 

 these caeca arising close together and appearing to be formed by the branching of 

 one caecum. In Urobenus the caeca are in the same segment. In the remarkable 

 Cryptodrilid Millsonia there are more than thirty pairs of caeca precisely like those 

 of Perichaeta. 



VI. Vascular System. 



The Oligochaeta agree with the Polychaeta in possessing a closed vascular 

 system, i.e. one having no communication with the body-cavitj' (coelom). Unlike 

 what is found in the Polychaeta there are no Oligochaeta known which have not 

 a vascular system, though the complication of the vessels belonging to this system 



