736 SUMMARY OF CUREEXT RESEARCHES RELATING TO 



when all the organs of the Nauplius are present in rudiment. Both 

 these pauses appear to be due to the want of further nutrient matter. 



One of the most important phenomena in the later development 

 of the larva is the development of the generative organs, the 

 rudiments of which appear very early. Here we have an example 

 of the development of the generative organs from the primitive 

 mesodermal cells ; the cause of this early ditferentiation is, in the 

 author's opinion, to be found partly in the phenomenon of partheno- 

 genesis. The organs which are unpaired in the adult are, in the 

 rudimentary stage, paired, and this shows that the paired stage is, 

 contrary to the opinion of Gegenbam-, the earlier condition. So 

 again, the organs are primitively ventral in position, although in the 

 adult they are dorsal ; and this it is interesting to compare with the 

 permanently ventral position of the ovary in Peripatus and the 

 Myi'iopoda. The large lateral compound eye of the Phyllopoda is 

 degenerate in the Copepoda, and the secondary portion of the cere- 

 brum comes into connection with the primary part of the growth of 

 this latter, and by the formation of new parts from the neighbouring 

 ectoderm. 



Organization of Trilobites.* — Mr, C. D. Walcott here gives a 

 most interesting account of the results of a seven years' investigation. 



It will be remembered that in 1873 Prof. L. Agassiz brought 

 forward what he believed to be ample evidence of the presence of 

 crustacean legs in an Asaphus platycejjlmlus. After a historical re- 

 view of the opinions on the matter the author gives his own account 

 of the structure of the Trilobite. Beneath the dorsal shell we find 

 (1) the ventral membrane, (2) the intestinal canal, (3) the appen- 

 dages beneath the head, (4) the appendages of the thorax and 

 pygidium, and (5) the respiratory apparatus. The fij'st of these 

 was thin and delicate, and was strengthened in each segment by a 

 transverse arch, to which the appendages were attached. The intes- 

 tinal canal is but rarely seen ; if one specimen in a hundred shows it 

 we have a " large proportion." The visceral cavity was usually 

 filled with calc spar, and all vestiges of the canal or any other organ 

 obliterated. The most interesting point is that the appendages of 

 the body are found to differ slightly in the genera Cahjmene 

 and Ceraurus. Of the latter genus the author gives an illustration 

 which is the finest of the Trilobites " as yet known." It would seem 

 to be certain that there is a " series of jointed legs extending from 

 the cephalic shield beneath the thorax and pygidium to the posterior 

 segment of the latter ; that, as far as known, they were ambulatory, 

 and formed of six or seven joints ; that to the basal joint there were 

 attached an epipodite and branchia ; and that, from the proof we 

 now have, there is little doubt but that the appendages beneath the 

 pygidium did not vary essentially from those of the thoracic region." 

 The branchial appendages consist of two series, attached as branchias 

 to the basal joints of the legs, and as branchial arms or epipodites. 

 The former appear under three forms ; the epipodite was formed 



* Bull. Mus. Comp. Zool., viii. (18S1) pp. 191-22i (6 pis.). 



