OKGANS OF SENSATION. 453 



anticipate the existence of the heart in the position indicated by the inde- 

 fatigable Neapolitan zootomist. The lateral appendages (fig. 232, d d d) 

 are most probably proper branchial organs, but, perhaps, not exclusively 

 the instruments of respiration, since the numerous cirri no doubt co- 

 operate in exposing the blood to the action of the surrounding medium, 

 a function to which they are well adapted by their structure and in- 

 cessant movements ; especially as each cirrus is seen under the micro- 

 scope to be traversed throughout its whole length by two large vascular 

 trunks, one apparently arterial, and the other of a venous character. 



(1171.) Judging from the peculiar conditions under which the Cir- 

 rhopods exist, it would only be natural to suppose that to creatures so 

 circumstanced, the possession of the organs of the higher senses would 

 be a useless incumbrance, seeing that they are apparently quite incapable 

 of holding communication with the external world ; nevertheless, from 

 the recent discoveries of Professor Leidy* and of Mr. Darwin f, they are 

 found to be by no means destitute in this respect. In Lepas fascicularis, 

 Mr. Darwin detected two nervous filaments, derived immediately from 

 the front of the two supra-cesophageal ganglia, which were found to 

 terminate in two small, perfectly distinct, oval masses, which are not 

 united by any transverse commissure. From the opposite ends of these 

 two ganglia, smaller nerves are derived, which, bending inwards at 

 right angles, communicate with the ocular apparatus, which, although 

 apparently consisting of a single mass, is, in reality, composed of two 

 eyes united together ; or, in other words, although in outline the eye 

 appears single, two lenses can be distinctly seen at the end, as well as 

 two pigment- capsules, which are deep and cup-shaped, and of a dark 

 reddish-purple hue. This double eye, in all the genera examined, is 

 seated deep within the body : it is attached by fibrous tissue to the 

 radiating muscles of the lowest part of the oesophagus, and lies actually 

 on the upper part of the stomach ; consequently a ray of light to reach 

 the eye has to pass through the exterior membrane and underlying 

 corium and to penetrate deeply into the body. In living sessile Cirri- 

 peds, Mr. Darwin observes, vision seems to be confined to the percep- 

 tion of the shadow of an object passing between them and the light ; 

 they instantly perceive a hand passed quickly at the distance of several 

 feet between a candle and the vessel in which they may be placed. 



(1172.) In the outer maxillae, at their bases, where they are united 

 together, but above the basal fold separating the mouth from the body, 

 there are, in all the Lepadidae, a pair of orifices, sometimes seated on a 

 slight prominence, or else on the summit of flattened tubes projecting 

 upwards and towards each other. Each of these orifices leads into a deep 

 sac lined by pulpy corium, and closed at the bottom, over which a nerve 

 of considerable size is distributed. That this closed sac is an organ of 



* Proceedings of the Academy of Natural Sciences of Philadelphia, Jan. 1848, 

 t Monograph on the Subclass Cirripedia, by Charles Darwin, F.E.S., 1841, 



