114 Descriptions of Preparations. 



in Myriapoda, Geophilus, Lithobius and Scolopendra ; and finally, that of 

 their having an all but independent annular segment, as well as a pe- 

 duncle, developed for their support in Squilla, appear to justify us in 

 regarding the eyes as either being or, when sessile, as representing ar- 

 ticulated appendages, and, by consequence, a distinct cephalic segment z. 

 The apparent paradox of speaking of the eyes, which ordinarily have a 

 more or less completely dorsal position as homologous not with such 

 dorsal outgrowths as the wings of insects or the shells of certain Crus- 

 tacea, but with the ventrally placed articulated appendages, is to be 

 justified by the history of the development of the pro-cephalic lobes, 

 which at an early period are bent upwards at a right angle to the rest 

 of the blastoderm, and even backwards, so that the roof of the skull, 

 which is really a sternal, appears to be a tei-gal surface. Taking then 

 the eyes as indicating one segment, and the two pairs of antennae and 

 the three pairs of jaws as indicating five segments, we find that the 

 typical number of segments in the head of the Arthropod amounts to 

 six. The segments anterior to the maxillae, together with that part of 

 the body which ultimately becomes the swimmeret, in the Astacus, 

 Fritz Miiller has called the ' primitive body,' as being that which makes 

 up the 'Nauplius' form of larva, and which carries the sensorial appa- 

 ratus, as, for example, the ear, which is lodged ordinarily in the scale 

 of the second pair of antennae, but sometimes in the ' uropodos,' as in 

 Ifysis. The sections of the body which are intermediate to these 

 extreme points, he divides into a ' fore-body,' which corresponds to what 

 is here spoken of as ' thorax,' and which is second in order of develop- 

 ment ; into a ' hind-body,' the ' post-abdomen,' deducting the sixth 

 segment, which is third in order of development ; and finally the ' middle 

 body,' the ' abdomen' in the language here employed, which in Crustacea 

 always puts forth limbs immediately after its segments are developed, 



127. The demonstration of the points relating to the brain of AmphipoJa, is made 

 much more easy if acetic acid is added to the alcohol employed for hardening the 

 specimens to be dissected. 



^ Two pairs of articulated appendiages have been observed in the larval forms of 

 Cirripedia anteriorly to the superior pair of their natatory antennae ; and if the 

 posterior of these be taken as equivalent to the ' olfactory filaments ' of the superior 

 antennae, the anterior, the 'larger antennae' of Darwin, the so-called 'horns of the 

 carapace' would still indicate the presence of a third pre-mandibular segment. For 

 an account of these outgrowths, see Gerstaecker, Klassen und Ordnungen des Thier- 

 reichs, Bd v., p. ,so8 ; Spence Bate, Ann. and Mag. Nat. Hist., Ser. ii., vol. viii., 

 1851, p. 327; Fritz Miiller, Archiv. fur Naturgeschichte, 1862, p. 7, 1863, p. 25, 

 note 2 ; Darwin, Lepadidae, i85i,p. 9; Balanidae, 1854, p. 105. 



