778 Transactions of the Society. 



Holopediiim gihherum Zaddach (plate XI.). 



The chief feature of this Entomostracan (fig. 1), and one 

 which cannot fail to make it of great interest, is that it is entirely 

 enveloped in a large gelatinous case, which, on account of its 

 extreme transparency and equal density with water, is almost 

 invisible when swimming, although when brought into air it is at 

 once apparent. This has been found with no other species, and 

 belongs to Holo2Jedium alone. When noticed by Zaddach, who 

 first discovered the species in 18.J5, he considered it as a disease 

 from which the animal would soon die. This view has, however, 

 been proved to be incorrect, from the fact that it is never found 

 without it, and that it always bears the same definite form. On 

 close examination it is seen to consist of two portions: a dorsal 

 and a ventral. Of these, the former is the larger, extending round 

 the anterior and dorsal portions of the animal for a distance about 

 equal to the diameter of the animal itself. The ventral is in the 

 form of two flaps which are fixed to the shell-valves and project 

 beyond them so as to cover the otherwise unprotected legs and 

 post-abdomen. Between these flaps is a sufiicient space for the 

 free movement of the feet, so necessary for respiration. 



The head is small, conical, and slightly bent forwards ; concave 

 anteriorly. The shell is strongly arched behind in the form of 

 a hump, thus making a large chamber for the reception of the 

 eggs, of which there are often as many as twenty at a time. The 

 valves of the shell are small, and leave the extremities of the feet 

 and the post-abdomen uncovered. They are widely open at their 

 ventral border, and the shell tapers back to a sharp edge at the 

 dorsal border, presenting the form of a V when looked at anteriorly. 

 At the posterior junction of the shell-valves there is an obtuse 

 point corresponding to the terminal spine which is generally found 

 in the Daphniadse. 



The diameter of Eolopedium is about ■ 1 in., exclusive of its 

 gelatinous case, with which it is as much as • 2 in. It is deli- 

 cately transparent, and every muscle and organ can be easily 

 distinguished. 



The eye is situated in the anterior portion of the head. It 

 is composed of a central pigment-mass surrounded by spherical 

 lenses, and is small in comparison with those of allied species. A 

 short distance below it is the pigment-spot, or nauplius eye, which 

 is unusually large, being about l-3rd the size of the eye. The 

 brain is situated below the eye and near the pigment-spot ; it is 

 a pear-shaped body, from the anterior portion of which a large 

 bundle of nerves runs to the eye. 



