III. MALACOPODA 399 



Pycnogonida (Pantopoda). 



These marine animals have a cyhndrical body, with a tubular proboscis in 

 front and an abdominal appendage behind, and four pairs of very long legs. In 

 front of the legs is a pair of small chelate appendages and usually a pair more 

 like pedipalpi. In the male there is an additional pair of 'ovigerous legs' to 

 Avhich the eggs are attached after being deposited by the female, thus giving a 

 total of seven appendages, a number not reached in any arachnid. Diverticula 

 of the stomach extend into the legs; a heart is present, but respiratory organs are 

 lacking. The Pycnogonids, which creep slowly over seaweeds and hydroids, 

 may be (i) a distinct group of arthropoda, or (2) modified arachnids, or (3), 

 and less probable, Crustacea. Nymphon* Pkoxichilidmm* Colossendeis.* 



Class III. Malacopoda (Protracheata). 



These forms, including only a single family Peripatid^, show a 

 strange mixture of annelid and arthropodan (or 'tracheate') characters, 

 so that they are usually regarded as representatives of the stock, early 

 separated from the annelids, from which the Insecta have descended. 



Fig. 435. — Peripatus capensis (from Balfour, after Moseley). 



They recall the annelids by the nephridia, which begin by a closed 

 vesicle (reduced ccelom), pursue a short course, and expand into a urinary 

 bladder before opening at the bases of the legs (fig. 436, so). On the 

 other hand, they possess tracheae, long unbranched tubes which arise in 

 numbers from the spiracles, which are irregularly distributed in each 

 somite {tr). 



Each segment of the soft-skinned body, which shows no external 

 ringing, bears legs, each terminated by claws. These legs resemble the 

 annelidan parapodia in not being jointed and not sharply separated from 

 the trunk. The head is provided with three pairs of appendages; a pair 

 of ringed antennas, a pair of mandibles, which lie in the oral cavity, and 

 a pair of mouth papillas, at the tips of which are the openings of the shme 

 glands, the sticky secretion of which is squirted out and serves to capture 

 insects {sd) . 



The nervous system consists of a pair of cerebral ganglia (og), sup- 

 plying the antenna; and a pair of very primitive eyes; and a pair of ventral 

 cords (bm), swollen slighdy in each segment, which connect dorsal to the 

 anus and are connected in the trunk by numerous non-segmental com- 

 missures. The muscles are of the smooth variety. 



