72 



Occurrence. Some few specimens of this form were found, many years 

 ago, at Christiansund, west coast of Norway, in a depth of about 30 fathoms. 

 This is the only place on the Norwegian coast where I have as yet met with it. 



Distribution. Liverpool Bay (Thompson). 



Section 3. 



Heterarthrandria. 



This section comprises the greater number of the hitherto known Calanoids, 

 and among them are some of the most highly organized forms. Whereas the 2 

 preceding sections only contain marine forms, the present one is represented both 

 in the sea and in inland-lakes and ditches ; some forms being also found in more 

 or less brackish water. The name of the section refers to the very dissimilar 

 appearance of the 2 anterior antenna? in the male, one of them being much of 

 same structure as in the female, whereas the other, as a rule the right one, is 

 peculiarly transformed, constituting a powerful grasping organ, by the aid of 

 which the female is seized during copulation. This prehensile antenna? exhibits 

 beyond the middle a very movable articulation, which admits the terminal part to 

 be doubled back upon the adjoining part, the latter being, as a rule, greatly 

 tumefied and traversed by a strong muscle, which acts upon the terminal part. 

 Thereby the said antenna becomes, as it were, divided into 3 successive sections, 

 generally well defined from each other: a basal, a median, and a terminal one. 

 More rarely the geniculate character is less pronounced, for instance in the genus 

 Acartia; but in the far greater number of forms it is very conspicuous, so as 

 at once to make these Calanoids recognizable from those belonging to the 2 pre- 

 ceding sections. The 5th pair of legs are always present in both sexes, though 

 in the female they may be much reduced in size. In some cases these legs are 

 natatory, like the preceding pairs; but in the male the outer ramus is always 

 somewhat transformed, and at any rate in one of the legs, generally the rigth 

 one, pronouncedly prehensile in structure. No difference whatever is found in 

 the structure of the oral parts in the two sexes, and, on the whole, the general 



