22 NEMERTINES OF PLYMOUTH SOUND. 
21. N. Negsu, Oersted, McIntosh. 
Like the last, this species abounds on the Breakwater, and is 
scarcely found elsewhere. 
The generative organs are ripe from March to October. Large 
numbers of eggs have been laid in my dishes, but they have always 
died without segmenting, though numbers of ripe males were present. 
Artificial fertilization has been equally a failure. 
Like the last, this species is widely distributed, ranging from 
Iceland to the Mediterranean. It is rare at Naples according to 
Hubrecht. 
Family MALACOBDELLIDAI, v. Kennel. 
Genus MALACOBDELLA, Blainv. 
22. M. crossa, Blainv. 
Several specimens of this interesting parasite have been obtained. 
In all cases they have been found in the branchial cavity of Cyprina 
islandica. In only one case have I examined one of these molluscs 
without finding a specimen, and in no case has more than one been 
found in a single Cyprina. 
This species lives well in captivity in spite of its parasitic habits. 
One specimen has lived in a vessel of water submerged in a tank 
for more than three months, and is still apparently healthy. It 
moves slowly about the sides of the vessel, to which it adheres by its 
sucker. 
In the autumn many ripe females were found, which subsequently 
laid unfertilized eggs, but no ripe males have been seen. 
Order 4, HETERONEMERTINI, Biirger. 
The lateral nerves situated, as in the Protonemertini, outside the 
circular muscular layer. Their apparent position, however, is 
different, owing to the appearance of a new muscular layer (longi- 
tudinal) which has inserted itself between the basement membrane 
and the circular layer. The lateral nerves are situated between this 
layer and the circular layer. 
The body-wall consists of epidermis, cutis, outer longitudinal 
muscular layer (new), a circular muscular layer, and an inner longi- 
tudinal muscular layer. 
Mouth situated behind brain. 
Proboscis without a stylet. 
