1316 CRUSTACEA. 



Fam. II. Caligid^e. — Corpus late depressum, peltatura, segmento 

 antico pergrandi. Antennae lmae breves; saepius 2-articulatse, raro 

 3-articulatae, 2dae corpore tectae. Pedes lmi graciles, 2di prehen- 

 siles vel ancorales. Ova externa in tubis gesta. 



Fam. III. Dichelestid^e. — Corpus angustum, segmento antico parvo. 

 Antennae lmae breves, 5-10-articulatae; 2dae fere frontales ultra 

 caput exsertae. Pedes lmi graciles, 2di prehensiles. Ova externa 

 in tubis gesta. 



We follow these general remarks on the relations of the Caligoidea 

 and their subdivisions, by some details with reference to their structure 

 and their homologies. 



Segments of the body. — In the Corycaeidae, among the Cyclopoidea, 

 the cephalo thorax consists of one large anterior segment, and three 

 posterior; and rarely there is another articulation, though less dis- 

 tinct, across the posterior part of the first of these segments. Each 

 of the three posterior segments bears a pair of natatories, that is, the 

 second, third, and fourth pairs of these organs ; and when another seg- 

 ment is present in the body, it bears the first pair of natatories. This 

 same structure is exhibited among the Caligoidea. In the species of 

 Pandarus and Specilligus, Plate 95, there are four cephalothoracic seg- 

 ments distinct, the last two of the body alone being abdominal. One 

 of the four is a large anterior segment, and the other three are pos- 

 terior segments, pertaining to as many pairs of natatories. In 

 Nogagus (Plate 94, fig. 9), the first of the three posterior is not dis- 

 tinct; and in the species of Caligus (fig. 1 a', and 1 h, Plate 93), only 

 the last is sej)arate, the others being coalesced with the anterior seg- 

 ment. Caligus, however, presents the semi-articulation alluded to, 

 just anterior to the first pair of natatories. The same is probably 

 present in Specilligus, although not shown in the figure, and if so, this 

 sj)ecies has the full number found in any Sapphirina. Moreover, all 

 the four posterior pairs of legs are true natatories in Specilligus, while 

 in Caligus the third pair is an apron-like appendage, and the fourth 

 is slender subpediform. 



In Argulus, we find the four segments distinct (fig. 2 a, Plate 94), 

 and also traces of the preceding or medial articulation. 



The bizarre forms of some Caligoidea arise from the enlargement or 



