OF THE PARASITIC LSOPODA. 
509 
early on the dorsal surface of the embryo, and was supposed by its discoverer to 
represent the dorsal organ of the Crustacea. 
Leuckart is stated to have described a similar structure in the Pentastomidse, but, 
unfortunately, I have been unable to refer to his paper. Various speculations have been 
put forward with reference to this organ, the most important of which will be found in 
two papers by Dr. Dohrn — one in the ‘ Journal of Anatomy and Physiology,’ 2nd series, 
vol. i., 1868, and the other in the £ Jenaische Zeitschrift fur Naturwissenschaft,’ bd. v., 
1 87 0 — and in a paper by M. Emil Bessels, ‘ Jenaische Zeit.,’ bd. v., 1870. In the two 
last papers full references are given to the extensive, literature of this organ. A 
lengthy discussion of these theories would be out of place here, but I may say that 
they all agree in regarding the dorsal organ, including the leaf-like appendages of 
Asellus, as being rudiments of some ancestral structure, and no longer of any use to 
their possessors. There is, no doubt, a good deal to be said for this view ; and in cases 
such as Cwtna, Praniza , and more especially the Spiders, where the organ is never 
large and soon disappears, it may well be regarded as a rudiment. But in the case of 
Asellus, Oniscus, and Cymothoa, the organ attains a considerable size, and its pro- 
duction must be a great tax on the resources of the egg — so great, indeed, that it is 
difficult to believe it would have retained such dimensions unless it performed some 
important function. It seems possible that the great variations in structure which it 
presents in different animals may be explained by supposing that it is really the 
remains of some lost structure, and that, in some cases, it is useless and has nearly 
disappeared, while in others it has been taken advantage of to perform various special 
functions, and has thus become modified in various ways. 
I will now return to the development of Cymothoa. 
Sections of an embryo at stage II. show that the ventral wall is formed of a single 
layer of columnar cells, beneath which are a few scattered cells more or less easily 
distinguishable as a distinct layer. From the columnar epiblast cells will be developed 
the epidermis, the central nervous system, the eye, and the fore- and hind-guts, 
together making up practically the whole gut of the adult. From the lower layer 
of cells the muscles, connective tissue &c., the liver, and a temporary mid-gut 
(Dottersack) surrounding the yolk, will be developed. 
Fig. 5 is a tranverse section through the thorax ; in the centre a slight hollow is 
seen, formed by the lateral parts being somewhat raised above the general level of the 
surface of the egg. The hollow is the cause of the dark mark running down the 
ventral line (fig. 3) ; from the raised lateral portions the limbs will be developed. 
At the edges of the Keimstveif the columnar epiblast cells (Eb.) alter in character 
rather rapidly, and pass into the flattened cells spreading over the yolk. 
At the anterior end of the embryo the epiblast is thickened on each side of the 
median line, and consists of several layers of cells (fig. 6). It is from these two lateral 
thickenings that the cerebral ganglia and eyes will be developed. In longitudinal 
sections (fig. 7) the segmentation of the body is indicated by the epiblast being thrown 
