CRUSTACEA. 



415 



ing much less from the type of the Thoracica than that of Cryptophialus 

 and Kochlorine. 



Sacculina leaves the egg as a Nauplius (fig. 233 A) which differs from 

 the ordinary type mainly (1) in the large development of an oval dorsal 

 shield (cp) which projects far beyond the edge of the body, but is provided 

 with the typical sternal horns, etc. ; and (2) in the absence of a mouth. 

 The Cypris and pupa stages of Sacculina and other Rhizocephala (fig. 233 B) 

 are closely similar to those of the Thoracica, but the paired eye is absent. 

 The attachment takes place in the usual way, but the subsequent meta- 

 morphosis leads to the loss of the thoracic feet and generally to retro- 

 gressive changes. 



OSTEACODA. 



Our knowledge of the development of this remarkable group is entirely 

 due to the investigations of Clans. 



Some forms of Cythere are viviparous, and in the marine form Cypridina 

 the embryo develops within the valves of the shell. Cypris attaches its 

 eggs to water plants. The larvfe of Cypris are free, and their development 

 is somewhat complicated. The whole development is completed in nine 

 ecdyses, each of them accompanied by more or less important changes in 

 the constitution of the larva. 



In the earliest free stage the larva has the characters of a true Nauplius 

 with three pairs of appendages (fig. 234 A). The Nauplius presents how- 

 ever one or two very marked secondary characters. In the first place it is 



B 



A 



FIG. 234. Two STAGES IN THE DEVELOPMENT OF CYPEIS. (From Glaus.) 

 A. Earliest (Nauplius) stage. B. Second stage. 



A'. A". First and second pairs of antenna?; Md. mandibles; OL. labrum; 

 MX', first pair of maxilla?; /". first pair of feet. 



completely enveloped in a fully formed bivalve shell, differing in unessential 

 points from the shell of the adult. An adductor muscle (SAf) for the shell 

 is present. Again the second and third appendages, though locomotive in 

 function are neither of them biramous, and the third one already contains 

 a rudiment of the future mandibular blade, and terminates in an anteriorly- 

 directed hook-like bristle. The first pair of antennae is moreover very 

 similar to the second and is used in progression. Neither of the pairs 

 of antennas become much modified in the subsequent metamorphosis. The 



