286 CLAUS, ON THE 



work I must refer in tlie explanation of the present abnormal 

 instance. This differs^ chiefly, in the number of the somites 

 and their appendages from the normal arrangement in the 

 fully formed Cyclopida, inasmuch as in the cephalo-thorax the 

 fourth thoracic ring, and the appendages belonging to it, are 

 entirely wanting. The abdomen, on the contrary, preserves 

 its full number of somites, and in its whole structure renders 

 the specific identity of the form with C. serrulatus probable ; 

 a supposition which is also supported by the size of the 

 caudal setae. On the other hand again, the first pair of 

 antennae is so short and compressed that the animal appears 

 far rather to belong to a form arising in the cycle of develop- 

 of a species of Cyclops characterised by seventeen -jointed 

 antennae. These organs possess only eleven joints, and, in 

 fact, of the same proportionate size by which, at the stage of 

 development when they consist of eleven rings, the first pair 

 of antennae is characterised (1. c, tab. ii, fig. 32). The three 

 pairs of feet, of which the first arises from the common 

 anterior division of the cephalo-thorax, support, it is true, 

 double branches ; but, nevertheless, appear to correspond 

 in the degree of development, since the branches are com- 

 posed of only two rings (fig. 2). The rudimentary pair of 

 feet is indicated by a simple hook, svipporting a single 

 seta, and thus differs essentially from the same part in C. 

 seri'ulatus. 



Although the deficiency of a thoracic somite and pair of 

 feet is in itself sufficient to indicate that the abnormal con- 

 dition must have arisen in an early stage of development, 

 the incomplete larval state of articulation in the segmental 

 appendages which do exist places it beyond doubt that we 

 have to do with an instance of arrested development. But 

 when we call to mind the process of development through 

 which the young Cyclops must pass after it has gone through 

 the Nauplius-like larval condition (1. c. p. 70, the tabular 

 summary), and remember the morphological characters 

 which are presented in the successive phases in the articula- 

 tion of the appendages appropriate to each stage in the seg- 

 mentation of the body, we are led to refer for the explanation 

 of the form now before us to deviations arising in the very 

 earliest stages of its development. For even in the immature 

 form, characterised by the existence of only five somites, we 

 find rudiments of parts which are equivalent to the absent 

 fourth thoracic somite and- its pair of appendages. These 

 parts, therefore, must either have been entirely wanting, or 

 at the next sloughing of the integument, accompanied with 

 the simultaneous failure of the new differentiation, instead of 



