APTOGONUM. 



ARACHNIDA. 



surface of the thallus, either immersed, super- 

 ficial or elevated on peduncles. They con- 

 tain the thecae or spore-cases. For the 

 structure, see LICHENS. 



Fig. 28, Dirina Ceratonia, natural size. 

 Fig. 29, several apothecia magnified about 

 5 diameters. Fig. 30, part of a vertical 

 section through an apothecium, showing theca 

 with spores, together with barren thecae or 

 paraphyses, magnified 200 diameters. 



APTOGONUM, Ralfs. A genus of Des- 

 midiaceae. 



Char. Filament elongated, triangular or 

 flattened ; joints bicrenate at the free mar- 

 gins ; an oval foramen between the joints. 



Kiitzing and others place this organism in 

 the genus Desmidium, where it might very 

 well have remained. 



1. A. Desmidium. Joints in front view 

 quadrangular, broader than long. 



a. Filaments triangular, regularly twisted, 

 crenatures rounded; length of jointsl-1500", 

 breadth 1-1000" (PI. 10, fig. 55, front view; 

 fig. 52, side view). 



/3. Filaments flattened; crenatures shal- 

 lower and slightly angular. 



2. A. Bailey i. Filaments not crenate ; 

 joints about equal in length and breadth. 

 American. 



The latter cannot be retained in this 

 genus, unless the characters be altered, on 

 account of the absence of the crenatures. 



BIBL. Ralfs, Brit. Desmid. pp. 63, 208. 



APUS, Scop. A genus of Entomostraca, 

 of the order Phyllopoda, and family Aspide- 

 phara. 



Char. Head, body and greater part of the 

 abdomen covered by a shield-like carapace, 

 which is deeply notched behind ; eyes two, 

 sessile and approximate ; a single pair of 

 minute, short, styliform and 2-jointed 

 antennas ; feet, sixty pairs, the first pair 

 furnished with three long, jointed branches, 

 extending beyond the carapace, the rest 

 branchial ; body composed of numerous 

 rings ; two long-jointed caudal appendages. 



1. A. cancriformis. Aquatic; brownish- 

 yellow ; length 2^ inches. 



2. A. productus. Not British; an elon- 

 gated oval lamina between the two caudal 

 appendages. 



BIBL. Baird, Brit. Entom. p. 18. 



AQUATIC, This term is used through- 

 out this work to signify an inhabitant of 

 fresh water as opposed to marine, inhabiting 

 the sea. 



ARACHNIDA. A class of animals, con- 

 taining the spiders, scorpions, &c. 



Char. Head united with the thorax, 

 forming a cephalothorax ; antennae none; 

 eyes simple (ocelli) ; legs eight, jointed. 



The integument of the Arachnida is usually 

 soft and leathery, rarely horny or brittle, and 

 consists principally of chitine. Two layers 

 may usually be distinguished, an outermost 

 or cuticle, which is the firmest and strongest, 

 and not unfrequently exhibits a cellular 

 appearance in the extremities and the cepha- 

 lothorax. The cuticle of the abdomen of 

 the Araneae, Acarina, &c., presents very 

 beautiful wavy or undulating lines, some- 

 times surrounding the roots of the hairs, &c. 

 concentrically, and arising, in some cases at 

 least, from the existence of folds (PL 2. fig. 4 



6 5). The cuticle of the Arachnida is fre- 

 quently covered with warty and bulbous 

 excrescences, bristles and simple or feathery 

 hairs, and sometimes with scales. 



The innermost cutaneous layer consists of 

 a very delicate and almost colourless mem- 

 brane, of a finely granular or fibrous appear- 

 ance; closely beneath which is situated a 

 layer of pigment granules and cells, which 

 are visible through the general integument, 

 and to which the beautiful colours of many 

 of the Arachnida are owing. 



The organs surrounding the mouth vary 

 in structure in the different families. In the 

 Spiders, two mandibles are situated at the 

 front of the head. These consist of two 

 joints, a basal very thick one (PL 2. fig. 6 a & 



7 ), and a terminal curved and sharply 

 pointed one (fig. 6 b & 7 b). The latter is 

 traversed by a canal terminating at its apex, 

 through which the secretion of a poison- 

 gland passes into any body transfixed by the 

 claw. These mandibles are perhaps, strictly, 

 modified antennas. Next come two maxillary 

 palpi (fig. 7 c), which do not diifer in struc- 

 ture from the legs, except in their tarsi 

 being composed of a single joint, generally 

 terminated in the females by a small hook, 

 but in the males of more complicated struc- 

 ture: the basal joints of these palpi are 

 enlarged and project forward, forming the 

 maxillae (fig. 7 d) ; in the scorpions, the 

 mandibles and maxillary palpi terminate in 

 pincers or forceps ; lastly a labium, situated 

 between the maxillae (fig. 7 e), and consisting 

 of a single piece. 



The mouth in the other famines is de- 

 cribed under the respective heads. 



The eyes are simple (ocelli, stemmata), 

 but they are absent in the parasitic Acarina ; 

 they consist of a simple arched cornea; a 

 spherical lens and a concavo-convex vitreous 



