14 EISEN 



cular arrangement of the penial duct in certain species of Limnodrilus, 

 where these muscles are spirally twisted around the duct. The ducts 

 enter the penial bulb always from the top, never from the side or from 

 the bottom, as, for instance, in Fridericia* Throughout their course 

 in the bulb the ducts are separated by strong muscles from the muscles 

 of the bulb, a character not found in the other subfamilies. The 

 structure of the bulb will be described more in detail under the genus 

 Mesenchytrczus. For a definition of the family we refer to the syn- 

 optic table of the genera. 



Genus Mesenchytraeus Eisen. 



Definition. Setae sigmoid, generally more numerous in the ventral 

 fascicles. Head-pore generally near the apex of prostomium. No 

 dorsal pores. Dorsal vessel rises posterior to clitellum, with cardiac 

 gland. Blood colorless or red. Brain generally truncate posteriorly, 

 generally broader than long. Nephridia with anteseptal, consisting of 

 the nephrostome, and with a deeply and irregularly pluri-lobed post- 

 septal, in which the ducts are wide and situated close together. No 

 salivary glands. Septal glands present. An atrium generally present. 

 Atrial and accessory penial glands present in many species. A single 

 median ovisac. One pair of sperm -sacs generally of large size. 

 Sperm-duct generally broad and short. Spermatophores present in 

 several species. Penial bulb when present contains muscular strands 

 which radiate from the base towards the periphery of the bulb. 



The above definition is slightly modified from the one given by 

 Michaelsen and Beddard. The points in question refer to the color of 

 the blood, to the presence of spermatophores in some species, and to 

 the nature of the penial bulb. An atrium or enlargement of the 

 sperm-duct is found in most species and may be said to be fairly char- 

 acteristic of the genus ; its absence is certainly the exception. In the 

 following we will consider in detail only such characters as are less 

 known. 



DETAILED DESCRIPTION. 



Brain. The form of this organ is less characteristic of the genus 

 than was supposed when the genus was established. The posterior 

 margin, while generally truncate posteriorly, is in many species con- 

 vex, while in a few it is even concave. But this convexity or con- 

 cavity is never as large as in the other genera, and coupled with some 

 other characteristics, is frequently a guide to the genus. These sup- 

 plementary peculiarities of the Mesenchytraeid brain are that it is 



