550 GEOFFREY SMITH. 
5. Tae RELATION OF THE SYNCARIDA TO OTHER MALACOSTRACA. 
The examination which has been made in the foregoing 
pages of the chief characteristics of the Syncarida has con- 
vinced us that they are the most generalised group of the 
Kumalacostraca. We have also seen that the structure of the 
living representatives of the division is practically identical 
with that of fossils which date, at any rate, from the Carboni- 
ferous period, so that they areamong the oldest Malacostraca, 
with the exception of the Nebaliacea, known. ‘Their 
generalised and primitive nature is not only shown by their 
great antiquity and by the possession of such characters as 
the freedom of all the thoracic somites, and the presence of 
eight joints in the thoracic limbs, but also by the fact of 
their combining in their own structure many of the distine- 
tive characters of the two divergent groups of the modern 
Eumalacostraca, viz. the Peracarida and the Eucarida. Thus 
we have seen that they possess the auditory organ, the 
alimentary canal and its glands, the spermatheca and copu- 
latory organs of the EKucarida, but at the same time the struc- 
ture of the heart and of the spermatozoa, the absence of a 
carapace, the direct mode of development and the maxillary 
oland point to Peracaridan affinities. 
The Synearida, therefore, represent to a great extent 
the ancestral form from which the Peracarida and the Hucarida 
have diverged. It has also been pointed out that the Syn- 
carida are closer to the Decapoda than to the other order of the 
Kucarida, viz. the Huphausiacea, so that the latter, so far from 
being the primitive Eucaridan type, must be considered as a 
specialised offshoot from the main Decapodan stem, which 
has lost certain primitive and ancestral characters. In fact, 
the only primitive character retained by the Kuphausiacea is 
the biramous structure of the thoracic limbs, and this has 
also been retained by the lower members of the Decapoda, 
and so is not particularly significant. 
In the followimg phylogenetic scheme we have given ex- 
pression to this idea. 
