SENSORY ORGANS. 167 



by its position at the anterior end of the body. The views of Ray Lankester 

 and Packard are supported most strongly by Kingsley's statements con- 

 cerning Crangon (No. 55) : not only was the procerebrum observed to originate 

 independently of the antennular ganglion, but the antennules and their pair 

 of ganglia were found in a distinctly post-oral position. In accepting this 

 view we should have, indeed, to assume, with Ray Lankester, a backward 

 wandering of the mouth. We must, however, await further researches as to 

 the structure of the Crustacean brain, and above all, as to the development of 

 the whole region now under consideration, before forming a decisive judgment. 



It is evident that, in a discussion as to the primary segmentation of the ante- 

 rior region of the Crustacean body, the question of the morphological value of the 

 first antenna comes to the front. Two alternatives are presented to us ; we must 

 either regard it as a true limb, although somewhat modified in shape, or else, 

 with Boas, deny that it has this significance, and consider it only as a stalked 

 sensory organ (similar to the stalked eyes). Only in the latter case can we 

 regard it as homologous with the primary cephalic tentacle of the Annelida. 

 We, however, see many reasons for regarding the first antenna as a true body- 

 appendage. We have only to recall the similarity between its position and 

 development and those of the latter in the embryo, and its use for swimming 

 purposes in the Nauplius stage and in many Entomostraca, in which the first 

 antenna is sometimes diverted to other purposes {e.g., climbing and sucking). 

 It is only in the higher Crustacea that this limb is distinctly set apart as a 

 sensory organ. If these considerations incline, us to place the first antenna in 

 the series of true trunk-limbs, we then have to ask whether the vestiges of the 

 primary cephalic tentacle, so common among the Annelida, are not to be sought 

 in some other structure. It is not difficult to assume this significance for the 

 so-called frontal sensory organs (Fig. 123, fs, p. 269) found in the young stages 

 of many Crustacea as paired peg-like or filamentous processes innervated from 

 the procerebrum. This view gains in probability by a comparison with 

 Peripatus, in whose embryos similar blunt processes have been observed, while 

 the antennae of Peripatus, according to their development and their relation 

 to the coelomic sacs, must be considered as modified trunk-limbs. If we held 

 this view, to which, naturally, we can only ascribe a hypothetical value, we 

 should be led to distinguish, with Ray Lankester, three sections in the 

 anterior part of the Crustacean body which contains the brain. There would 

 be one actual primary section, originally the only pre-oral section of the body, 

 with the procerebrum, the eyes, and the frontal sensory organs, and two sections 

 following posteriorly, trunk-segments drawn into the head (antennular and 

 antennal segments), for which we must assume an originally post-oral position. 

 We must, however, once more emphasise the fact, before accepting any one 

 of these views, that in discussing these questions we are dealing entirely with 

 hypothetical matters. 



D. Sensory Organs. 



Of the details of the development of the unpaired triple Nauplius 

 or Entomostracan eye* nothing is as yet known, but mention should 



* According to Claus {Kaiserl. Akad. Wissensch. Anz. Wien, 1891), the 

 Nauplius eye is composed of three inverted cup-shaped eyes, in which the 

 nerves enter the retinal cells from the side turned away from, while the rods 

 are directed towards, the pigment cups. A certain similarity with the median 

 eye of the Arachnoida is thus brought about. [Bernard (The Apodidae) 

 claims to have discovered paired rudiments for the Nauplius eye of Apus. — Ed.] 



