PYCNOGONIDAE : CRUSTACEA. 543 



and is composed of two ganglia fused. There are four well-developed 

 thoracic ganglia, and two others which are either rudimentary or aborted. 

 The species inhabiting shallow water have four eyes, those living below 

 400 fathoms rudimentary eyes without pigment, or none at all. The eyes 

 appear to be monostichous and monomeniscous. The alimentary canal 

 consists of an oesophagus dilated posteriorly, and containing a masticatory 

 apparatus ; of a mesenteron beset with cellular villi (= glands) and giving 

 off caeca which enter the proboscis in many instances, the mandibles and 

 limbs, but not the palpi and ovigerous limbs. Their number is not 

 regular apparently. There is a dorsal heart which is continuous dorsally 

 with the integument, open anteriorly and furnished with two pairs of lateral 

 ostia, and either a third pair or a single posterior aperture. Respira- 

 tion is cutaneous, and there are respiratory cavities or depressions 

 opening externally by a pore. The testis consists of two dorsal tubes 

 united posteriorly, and sending processes outwards into all the thoracic 

 limbs, or into the three or the two last pairs only. The ducts open externally 

 by a ventral pore on the tibial joints. The ovary either resembles the 

 testis, or the portion within the body becomes atrophied. The branches 

 in the limbs open in the same position as do the branches of the testis, 

 but there are no true ducts. Copulation takes place. The eggs are 

 fecundated externally, and are carried by the male on the ovigerous limbs, 

 by the female also in Nymphon brevicati datum. They are attached by the 

 secretion of special glands placed in the fourth joint of the limbs. The 

 three pairs of cephalic limbs are formed as a rule first, and the young may 

 be hatched in this condition. They remain attached to the ovigerous 

 limbs by a thread secreted by a gland lodged in the basal joint of the 

 mandible. The larvae of Phoxichilidium are entoparasitic in various 

 Hydrozoa. The affinities of the group are doubtful. 



The terms cephalothorax, mandible, palpi, &c. must not be taken as implying 

 a homology with the parts so named in Arachnida or Crustacea. 



Hoek, Challenger Reports, iii. 1880; Dohrn, Fauna und Flora des Golfes von 

 Neapel, iv. 1881. 



PHYLUM ECHINODERMATA. 



Coelomate Metazoa in wiiich the bilateral symmetry of the larva is 

 more or less completely replaced by a radial symmetry. Calcifications of 

 the integument form a mesodermic skeleton generally of great completeness, 

 and a special section of the primitive coelome developes into a water- 

 vascular system, which has a locomotor and often a respiratory function. 

 The five peripheral vessels of this system define five radii and five intervals 

 or interradii, the ambulacra arid interambulacra respectively. There is 

 a metamorphosis. 



