CHAPTER XXIII. 

 THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



I. The Cirripeds. 



The cirripeds are the only members of the acraniates in which the more 

 typical arthropod characters are retained. They present an extraordinary diver- 

 sity of form and structure, but many of their peculiarities, such as the enormous 

 cephalic stalk, the mantle, pigmyism, the absence or disappearance of appendages, 

 of sensory and alimentary organs, and of external segmentation, we shall see ex- 

 pressed in a more stable and permanent form in other members of the group. 



The Nauplius and the Naupula.— The young leave the egg like many 

 other primitive arthropods, in the nauplius stage, as a small, free swimming larva 



B 



Fig. 272. — Diagrams of a nauplius, based in part on Pedeschenko's figures of Lemaea; A, neural surface; B, haemal. 



with three pairs of appendages. (Fig. 289.) Its minute structure is doubtless 

 very similar to that of a parasitic copepod. The larva of the latter being better 

 known, it may be taken to illustrate the structure of the nauplius, the basic larval 

 form of the entire group of acraniates. 



In Lerneea branchiata, which has been carefully studied by Pedaschenko, 

 the nauplius (Fig. 272), is provided with a well developed brain, hr., frontal or- 

 gans, ol., rudimentary lateral eyes. I.e., a trioculate median eye, ^.e., and stomo- 

 dseal ganglia, st.g., very similar to those we have already seen in Branchipus, in 

 other phyllopods, and in Limulus. There is also a large cephalic navel, do. 

 (dorsal organ) that undergoes a characteristic degeneration and absorption by 

 the yolk cells. 



Behind the mandibular ganglion the nerve cords are widely separated, but 

 they unite again at the caudal end, forming the rudiments of the abdominal neuro- 

 meres, or ventral cord. The latter increases in length by the multiplication of 

 prominent telo-neuroblasts. A middle cord, m.ch., is clearly indicated. 



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