l8 EISEN 



SYNOPSIS OF SPECIES OF MESENCHYTR^EUS. 

 In order to facilitate the examination of the various species of this 

 genus, I have compiled the following table, based on a reexamination 

 of the old descriptions of such species as were previously known. It 

 need hardly be stated that in none of the older descriptions was the 

 structure of the atrium and its tributary glands referred to in detail. 

 This makes it necessary to base the arrangement of the species on some 

 other characters, as, for instance, on the presence or absence of di- 

 verticles of the spermatheca and upon their number. The largest 

 number of species belongs to the group with two diverticles. This 

 group may be further subdivided according to the nature and size of 

 the spermathecae. Other subdivisions are based on the presence or 

 absence of the glands accompanying the sperm-ducts. In the follow- 

 ing table I have enumerated several species which are insufficiently 

 described, but which are sufficiently well defined to be identified. 

 This refers to all species which have been described from dissections 

 only, the finer histology not having been studied. 



I. SPERMATHECA WITHOUT DIVERTICLES. 



1. Sperm-ducts thick and short. Penial bulb long and tapering. Small pe- 



nial glands confined to the bulb. No atrial and no accessory glands. Sper- 

 matheca twisted at the pore. Brain posteriorly strongly emarginated. 



I. M. unalaskce sp. nov. 



2. Sperm-ducts short and narrow. Spermatheca straight and of even thick- 



ness. Head-pore between prostomium and somite I. Body transparent. 

 Brain posteriorly slightly convex. Sperm-sac confined to XII. 



2. M. fenestratus (Eisen, '79). 



3. Sperm-ducts short and narrow. Penial bulb short and ellipsoidal. Sper- 



matheca straight and of even thickness. Brain posteriorly slightly emar- 

 ginated. Narrow part of sperm-funnel helix-like. 



3. M. falciformis Eisen, '79. 



4. Sperm-ducts short and broad, three or four times as long as the funnel. 



Spermatheca with an apical ampulla at the junction with the intestine. 

 Brain slightly emarginated posteriorly 4. M. flavidus Michaelsen, '87. 



5. Spermathecal pore not conspicuous. Spermathecae club-shaped. Body 



dark, pigmented, but not quite black. Brain posteriorly concave. Ice 

 worm from Malaspina Glacier 5. M. niveus Moore, '89. 



6. Setae, dorsal : 3 to 5 ; ventral : 6 to 9. Head-pore at apex. Brain square, 



posteriorly emarginated. Sperm-funnel about square, small. Sperm- 

 duct short. Spermatheca sac-like, folded, without diverticle ; connected 

 with intestine ; duct half as long as the ampulla. 



6. M. montanus Bretscher, '99. 



7. Sperm-duct short and thin. Spermatheca large, sac-like, not connected with 



intestine. Brain posteriorly deeply emarginated. Setae 3 to 6. Lympho- 

 cytes pointed oval, dark with large granules 7. M. tigrina Bretscher. 



II. SPERMATHECA WITH ONE DIVERTICLE. 



I. Spermatheca with a pear-shaped diverticle at the center. No enlarged 

 lateral setae. Sperm-sacs confined to XII. Head-pore at the apex of 

 prostomium. Setae : laterals 3, ventrals 5. Funnel short, and sperm- 

 ducts short and hardly convoluted 8. M. flavus Lev., '84. 



