826 Transactions of the Society. 



I. External Structure. 



The general appearance of Leptodora is shown in Fig. 2. 

 The body, which is long and straight, is of nearly the same 

 width thronghout. It measures from J inch to ^ inch in 

 length and about oV inch in greatest breadth. The division 

 into head, thorax, and abdomen is well marked. The head 

 (Fig. 1, a-h) is apparently of one piece, having no traces of seg- 

 mentation. In shape it is a rounded column, gradually expanding 

 towards the base, and slightly flattened on the ventral side. On 

 each side of the eye there is an antennule ( = the superior antennae 

 of Baird\ which is long and tapering in the male (Fig. 8), 

 but short and claviform in the female (Fig. 9). Just above 

 its junction with the thorax the head is covered, dorsally, by 

 a saddle-shaped shield (= cephalostegite of Huxley), beautifully 

 constructed of flat polygonal cells, each with a raised dot in the 

 centre of it (Fig. 1, e). Almost opposite this, on the ventral 

 surface, is the mouth, one of the most extraordinary I have ever 

 seen. In the Crustacea we are accustomed to find a complicated 

 arrangement of maxillae, palpi, and other hard structures. In 

 Leptodora we find, however, simply an upper and lower lip, soft 

 and mobile, the upper fitting over the lower like the two halves 

 of a duck's beak. When the animal requires to open its mouth 

 it calls into play a set of muscles which cause the upper hp to 

 wrinkle and curl upward in a manner which cannot fail to strike 

 the observer as comical. The lower lip, which is the same shape 

 as the upper, possesses little or no mobility ; it almost rests on a 

 kind of platform, formed by the top of the thorax (Fig. 1, h, upper 

 lip ; i, lower lip). 



On each side of the body, between the cephalostegite and the 

 thorax, are attached the antennae (= inferior antennae of Baird), 

 joined to the body by several wrinkled folds of the integument (Figs. 1 

 and 2,/). Each antenna consists of a large, broad, and slightly 

 flattened arm, divided at the end into two branches (see Fig. 3) of 

 four joints each. The arm is not setose, but the three distal joints 

 of the one branch bear nine, six, and nine setae respectively, and 

 the four joints of the other branch bear four, nine, five, and six 

 setae respectively. These are the usual numbers, but they vary 

 slightly in difierent individuals. Each seta (Fig. 4) is divided into 

 two, and occasionally three joints, and tapers ofi" to a very fine 

 point. The sides are delicately fringed with hairs of extreme 

 tenuity, so closely set that the whole forms a kind of oar almost 

 impervious to water; presenting, perhaps, the largest possible 

 surface, at the smallest expenditure of material. The antennae are 

 used solely as organs of locomotion, i.e. as swimming organs. 

 Their motion is chiefly perpendicularly up and down. Though 



