1883.] Hetcrogenetic Development in Diaptomus. 385 



This may be observed in the abdominal feet of Palaemon, the 

 mouth parts of Cyclopidse, 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 ; antennae 25-jointed, in the male 

 6 joints follow the hinge, the enlarged portion is not greatly thickened, the antennae 

 reach somewhat beyond the thorax ; mandibles with about nine teeth, the hrst of 

 which is large and divaricate, more or fewer of the following ones are emarginate ; 

 mandibular palp biramose, inner branch i-jointed, outer branch 3Jointed; maxilli- 

 peds not unlike Diaptomus but shorter and more strongly armed with curved spines; 

 (last feet of female i -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 

 foot 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 

 setae , 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 setae at the end, 

 thus is formed a powerful hand. 



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 

 are offered with some hesitation, as such a coalescing of a limb 

 with the abdomen has never been described. However I believe 

 the same thing takes place, though to a less degree, in Cyclops. 

 In C. midleri, for example, the fifth foot entirely disappears, leav- 

 ing 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 

 second ramus of the fifth foot By comparing Figs. 1 1 and 1 2 of 

 Plate v with i and 3, representing the corresponding parts in 

 Diaptomus. it will be seen that the theory advanced places all the 

 parts in the place demanded by the schema of the limbs in 

 Calanidae. 



Comparing the fifth leg of the female, Figs. 12-13, with the 

 preceding one, Fig. 20, it appears to correafpond with one leg only, 



