538 



COMPARATIVE ANATOMY 



CHAP. 



\ 



2. Embryonic envelopes have till now only been found in the Scorpionidce. The 

 embryonic envelope here, as in the Insecta, consists of 2 membranes, the outer repre- 

 senting the serosa, the inner the amnion of the Hexa- 

 poda. 



3. The formation of segments in the embryonic 

 rudiments takes place as a rule from before backward, 

 so that new segments are continually formed from the 

 terminal segment behind those already developed. Fre- 



;V%f~~" ~yyjjil quently, however, the segment bearing the chelicerfe, 



1 V " ^ ~ * and sometimes that bearing the pedipalps, appear only 

 after the formation of a few of the subsequent segments. 



4. The rudiments of the extremities seem in various 

 Arachnoidea to have different orders of succession. The 

 permanent extremities, with the exception of the cheli- 

 cerfe, which begin to form later, often develop simul- 

 taneously. In the Pseudoscorpionidce the rudiments of 

 the extremities are even said to be recognisable before 

 the marking off of the segments on the embryonic 

 rudiments. 



5. In all Arachnoidea, except the Linguatulidce, the 

 body is, in its embryonic condition, more richly seg- 

 mented than in the adult animal. The cephalo- 

 thoracic region especially shows embryonic metamerism. 

 This region consists at certain embryonic stages of a 

 cephalic or frontal lobe, in which the stomodseum and 

 the definitive oral aperture form, and of 6 subsequent 

 and thus post-oral segments, the 1st being that of the 



ciba 



FIG. 379. Embryo of a Scor- 

 pion, spread out flat, from the 

 ventral side (after Metschni- 

 koff). U, Frontal lobes; 1, 

 chelicerse; 2, pedipalps; 3-6, 

 the 4 pairs of legs ; aba, rudi- 

 ments of the abdominal limbs ; 

 pa, post abdomen. 



chelicerse, the 2d that of the pedipalps, while the 4 others are the segments of the 

 4 following pairs of extremities. In the abdomen also, even when there is no meta- 

 merism in the adult animal, .segmentation is to be recognised in the embryo. The 

 number of the embryonic abdominal segments in the various divisions of the Arach- 

 noidea, however, differs greatly. 



It is a specially important fact that the segment bearing the chelicerse is 

 in the embryo post-oral. No extremities develop on the frontal lobe, where, in the 

 Crustacea, Protracheata, and Antennata, the antennae form. 1 From this fact it follows 

 that there is no correspondence between the chelicerse of the Arachnoidea and the 

 antennae of the Antennata. 



The appearance of rudimentary abdominal limbs in the embryos of many 

 Arachnoidea has already been mentioned ; some of these rudiments disappear later, 

 some, however, are retained (e.g. the combs of the Scorpion, the spinning mammillae of 

 the Araneidce). 



6. "What has been said about the relation of the embryonic metamerism of the 

 body to the definitive metamerism of the same is also true of the nervous system. 

 Investigations show that in the Scorpionidce and Araneidce a pair of ganglia forms in 

 each embryonic segment. The embryonic pair of ganglia of the frontal lobe is the 

 rudiment of the supra-resophageal ganglion. In the first post-oral segment a special 

 ganglion for the chelicersB is developed, which only secondarily joins the supra- 

 cesophageal ganglion, forming with it the brain. In the Antennata and Protracheata, 

 on the contrary, the antennae are from the very first innervated from the preoral 

 supra-cesophageal ganglion. Each of the following embryonic segments except the 

 terminal segment in like manner possesses 1 pair of ganglia. The more or less 



1 See footnote on page 516. 



