1^8 MCMURRICH. [Vol. XI. 



o 



In later stages the ganglion G as well as the antennular 

 ganglion, and later still the antennary,fuse with one another and 

 with the cerebral ganglion to form the syncerebrum of the adult. 

 The occurrence of this supernumerary ganglion, whose presence 

 indicates the existence of a segment between the eye-bearing 

 and the antennular segments, has already been described by 

 Bumpus ('91) as occurring in the Lobster, and in that form 

 what may possibly be transient indications of a pair of appendages 

 corresponding to the ganglia are developed. I have not been 

 able to detect in the Isopods any indication of appendages, 

 but there can be no doubt that the ganglia I have described are 

 homologous with those seen by Bumpus. From the abstract 

 of Nusbaum's paper ('93) it seems that he has observed what he 

 takes to be a pair of praeantennular ganglia in Ligia, which are 

 not, however, identical with those I have found, having an 

 entirely different position and being formed by a splitting off 

 of a portion of the antennular ganglia. The praeantennular 

 ganglia of Homarus and those I have described here arise quite 

 independently of both the cerebral and the antennular ganglia, 

 and have, I believe, a segmental value. I have seen thickenings 

 in the region in which they are indicated by Nusbaum, but 

 what their significance may be I am not prepared to state. 



I do not intend to follow out the questions of homology which 

 the presence of a praeantennular segment suggests, especially 

 as they have already been discussed by Kingsley ('94). It seems 

 to me that further observations, especially upon the develop- 

 ments of the Entomostracan forms, are necessary before any 

 certain conclusions can be formulated. 



In front of the mouth in Fig. 70 two elevations {cp) are seen, 

 which later stages show uniting to form the upper lip, and 

 behind the mouth on a level with the mandibles two other ele- 

 vations {Mt) are developed, which give rise eventually to the 

 metastoma. There has been a certain amount of discussion 

 as to whether the metastomal elevations are to be regarded 

 as of metameric value and as having the same significance as 

 limbs. The evidence furnished by the Isopods points to a neg- 

 ative answer to this question, since no nerve-ganglion or 

 neuromere which can be assigned to them is distinguishable. 



