LiircnJ Slayas of Trilubites. — Beecher. 185 
An anal opening* is found in most nauplii, especially in 
those of the non-parasitic Crustacea, and in those in which 
this stage is normal and free-swimming. The protaspis, as 
representing a free-swimming larval stage of trilobites, there- 
fore, i)robably possessed an anal opening. 
The only character represented in the restoration which is 
accepted purely from analogy is the median un[>aired ej^e. 
This organ is almost universally present in the nauplius, and 
is regarded as a very primitive character wherever found. 
The next and last structures to be noticed are the free- 
cheeks and the beginnings of the paired eyes, as shown in 
plate X, figure 1 (//, <>c). Their existence has already been 
indicated in the descriptions and observations of the protaspis 
and its derived characters, and need not be repeated here. Ap- 
))arently the nauplius presents nothing homologous, unless 
possibly the frontal sensory organs of Apns. Balanu.s, Peneus, 
etc., may be taken as such. The paired eyes and frontal sen- 
sory organs are close together and seem to have some intimate 
connection, for, as the paired eye^ develop, the latter dwindle 
and disappear. Likewise in the trilobites the fi-ee-cheeks bear 
the visual areas, and may l)e ahnost wholly converted into 
eyes as in ^lhJ(/h'jiii {('i/c/"/>i/!/<'). 
The greater or less separation of the cerel)ral ganglia in the 
chsetopods and in some of the htwer crustacea leads to the 
idea that the free-cheeks in trilobites are the pleura of an oc- 
culiferous head segment, which otherwise is lost. If the hy- 
])ostoma is homologous with tht^ annelitl prostomiuni. as urged 
by Bernard'^ then tiu^ free-cheeks nuiy be considered as rep- 
resenting the second procejihalic segment, which is the num- 
ber re(|uired on the sujiposition that each ucuromere corres- 
I)onds to a somite. There is a se))arate neuromere to each 
nu^sodermic metamere posterior to the head, and from analogy 
we should expect that each neuromere in the head would 
represent an original segment, especially as it can be demon- 
strated that the head is composed oP consolidated or fused 
segments ( Kings ley'-'). 
Having thus shown the probal)le ventral structure of the 
protaspis, we are prepared to make some general observations 
on the larval type of moilern Crustacea known as the Xmi/i/in.^-. 
Before doing this it is well to emphasize again that there is 
