468 



AltACHNIDA. 



vided with chelicerae and palpi, or one kind of these organs) constitutes the mouth.* Both sexes have 

 eight feet, fitted for running ; but the females exhibit, besides, two false legs, situated near the anterior 

 pair, and only employed in carrying the eggs. These animals are marine, analogous either to Cyamus 

 and Caprellaf, or to the Arachnida of the genus Phalangium, with which Linnaeus united them. The 

 body is commonly linear, with very long legs, consisting of eight or nine joints, and terminated by two 

 unequal ungues, appearing only to form a single one, the smaller one being slit. The anterior segment 

 of the body, which replaces the head and mouth, forms a projecting tube, nearly cylindrical, or conical, 

 having a triangular or trilobed orifice at its extremity. It is furnished, at the base, with the chelicerae 

 and palpi. The former are cylindrical and linear, simply prehensile, 2-jointed, the terminal joint che- 

 liferous, with the lower finger, which is immoveable, sometimes very short. The palpi are filiform, 

 from 5 to 9-jointed, with a hook at the tip. Each succeeding segment, with the exception of the last, 

 supports a pair of legs ; but the anterior of those with which the head is articulated, bears, on the 

 back, a tubercle, on which is placed a pair of ocelli ; and on the under side, in the females alone, two 

 other slender legs, folded upon each other, and bearing the eggs, which are placed all round them in 

 one or two masses. The last segment is small, cylindrical, and pierced by a small orifice at the tip. 

 We can discover no vestiges of spiracles. M. Edwards, who has observed these animals in a living 

 state, tells us that he has seen, in the interior of the feet, lateral expansions of the intestinal canal, or 

 ccecums. I had also perceived the traces, under the form of blackish vessels, 

 in different Nymphons ; and hence I am induced to believe that these creatures 

 respire by the skin, — a peculiarity which would render the establishment of a 

 distinct order necessary, probably between the Arachnida and apterous para- 

 sitic insects. They are found amongst marine plants, under stones near the 

 beach, and occasionally also on the Cetacea. 



Pycnogonum, Brunn., Mull., Fabr., is destitute of chelicerae and palpi, and their legs 

 scarcely exceed the length of the body, which is proportionately shorter and thicker 

 than in the following genera. They live upon Whales. 



Phoxichilus, Latr., has no palpi, but the legs are very long, and they have two chelicerae. Pycnogonum 

 spinipes, O. Fabr., — Ph. aculeatum and spinosum of Montague, Transactions of the Lintuean Society, — Nymphon 

 femoratum of the Acta of the Society of Natural History of Copen- 

 hagen, 1797, &c. 



Nymphon, Fabr., resembles the last in the very narrow and ob- 

 long form of the body, the length of the legs, and presence of cheli- 

 cerae ; but they have moreover two palpi, composed of five joints. 

 N. grossipes, O. Fabr., Miiller, Zool. Dan. Compare, also, Leach, 

 Zool. Miscelt. vol. iii. 19, f. 1, 2. 



Ammothea, Leach {A. carolinensis, Leach), differs from Nymphon 

 in the chelicerae being much shorter than the mouth, the basal piece 

 being very small. The palpi are 9-jointed. 



[From the apparent absence of breathing pores, Latreille, in his ' 

 Cours d'Entomologie, forms these animals into a distinct order,— 

 Aporobranchia ; but Leach had previously given to them the ex- 

 pressive name of Podosomata. There are several British species 

 described by Dr. Johnston in the Magazine of Zoology and Botany, 

 No. iv., wherein several new genera are proposed. It will, however, 

 be necessary to change the names of some of them, as they are 

 already employed for genera of Crustacea. A still more extra- 

 ordinary genus, with ten legs, is described by Eights in the 

 Boston Journal of Natural History, under the name of Decalo- Fi s- 36 - _ Nymphon grossipes, and under side of its iieak. 

 poda australis.] 



Fig. 35. — Pjcnogonum balre- 

 narum. 



THE THIRD FAMILY OF THE TRACHEAN ARACHNIDA,— 



The Holetra (Hermann), — 

 Has the thorax and abdomen united into a mass, beneath a common epidermis. The thorax is at most 

 divided into two by a strangulation ; and the abdomen merely presents, in some species, the traces of 

 articulations, formed by foldings of the epidermis. The anterior extremity of the body is often ad- 



* The siphon of a large Phoxichilus, brought from the Cape by 

 Delalande, exhibits longitudinal sutures, so that it appears to me to 

 consist of a labrum, tongue, and two maxillae, all soldered together. 



The palpi are thence those of the maxilla*. 



t According to Savigny, they form the passage between the Arach- 

 nida and Crustacea. I place them in this situation with doubt. 



