E EAA. E i hc oH a. 
1883.] Heterogenetic Development in Diaptomus. 385 
This may be observed in the abdominal feet of Palæmon, the 
mouth parts of Cyclopidæ, but extends to the first pair of swim- 
ming feet in Diaptomus, and in this case involves all of them. 
The advantage of this arrangement, as well as its cause in the law 
of adaptation, is evident in the case of such animals as rely much 
upon a current below the body for food or the aeration of the 
blood. In Epischura the antennules rotate and create such a cur- 
rent past the mouth as is seen in other Copepods. 
Char. spec.—Cephalothorax imperfectly 6-jointed ; antennæ 25-jointed, in the male 
6 joints follow the hinge, the enlarged portion is not greatly thickened, the antenna 
reach somewhat beyond the thorax ; mandibles with about nine teeth, the frst of 
which is large and divaricate, more or fewer of the following ones are emarginate; 
mandibular palp biramose, inner branch I-jointed, outer branch 3-jointed; maxilli- 
peds not unlike Diaptomus but shorter and more strongly armed with curved spines; 
(last feet of female 1-branched with a straight claw terminating the distal segments, 
or) left foot obsolete and the other 2-branched, each branch 3-jointed, the right male 
of last pair is much more modified, its inner ramus is lamellate and curved in 
upon itself so as to make a grasping organ of curious form, in this office it is aided 
by two or more curved movable hooks which may probably be regarded as modified 
m the second joint of the abdomen bears on its left side an appendage of two 
Joints, the basal joint being flat and extending into a strong curved claw reaching to 
the base of the furca, while the second is slender and has two small sete at the end, 
thus is formed a powerful hand, j 
I regard this appendage as perhaps the terminal joint of the 
left of the last pair of feet ; indeed there seems to be some internal 
connection with the last thoracic segment, although externally 
none remains, the abdomen is otherwise quite normal and 
straight, 
These Suggestions with regard to the homologies of the organ . 
ane offered with some hesitation, as such a coalescing of a limb 
the abdomen has never been described. However I believe 
i. st thing takes place, though to a less degree, in Cyclops. 
| &. mulleri, for example, the fifth foot entirely disappears, leav- 
mg only two separate spines to indicate its position. It is sug- 
gested : that certain spines adorning the first segment of the abdo- 
Men in most Species of Cyclops may be rudiments of the missing 
— d Famitig of the fifth foot. By comparing Figs. 11 and 12 of 
re with I and 3, representing the corresponding parts im 
“aptomus, it will be seen that the theory advanced places all the 
rae in the place demanded by the schema of the limbs in 
Comparing the fifth leg of the female, Figs. 12-13, with the 
"8 One, Fig. 20, it appears to correspond with one leg only, 
