306 BKITISH ENTOMOSTRACA. 



three joints, the terminal one being provided at its ex- 

 tremity with several short setae. 



The inferior half of the body of the animal is composed 

 of four segments, the first of which is the largest, and has 

 on its ventral surface a pair of members exactly similar to 

 the last pair described, belonging to the cephalo-thorax. 

 The other segments decrease in size as they descend, and 

 the last is slightly lobed at the extremity, and furnished 

 with several setse. 



The smaller of the two forms of embryo differs in the 

 shape as well as the size of the cephalo- thoracic por- 

 tion, being ovate, the superior portion broader than the 

 inferior, and not expanded laterally. The lower half of 

 the body is narrower than the corresponding portion of 

 the first form, but consists of the same number of seg- 

 ments. 



The eyes and antennae are similar, but the latter organs 

 are proportionally larger. 



The foot-jaws are either wanting or so rudimentary as 

 not to have been observed, while the last two pairs of 

 members are narrower and shorter than those in the first 

 form. 



In general conformation we thus find the young of the 

 Nicothoe very similar to the young of the Cyclops, and 

 we can trace the gradual transformation of the different 

 parts of which the animal is composed. The eyes, mouth, 

 antennae, and three pairs of foot-jaws already exist, though 

 in their progress towards maturity they undergo consider- 

 able changes. We find, however, only two pairs of 

 swimming-feet, while in the adult there are five. The 

 two upper pairs become smaller, but assume a branched 

 form, and the three others are added dm-ing the animal's 

 progress towards maturity. The whole body consists, in 

 the young, of five segments, in the adult of ten ; five rings 

 are thus added during its growth, before it reaches the 

 full-grown state. 



In general form, the Ergasilidae appear to connect the 

 Cyclopidse with the Lerneadae ; but in habits and mode 



