448 PYCNOGONIDA, 



PYCNOGONIDA. 



The embryos, during the first phases of their development, are always 

 carried by the male in sacks which are attached to a pair of appendages 

 (the third) specially formed for this purpose. The segmentation of the 

 ovum is complete, and there is in most forms developed within the egg- 

 shell a larva with three pairs of two-jointed appendages, and a rostrum 

 placed between the front pair. 



It will be convenient to take Achelia la?vis, studied by Dohrn (No. 

 536), as type. 



The larva of Achelia when hatched is provided with the typical three 

 pairs of appendages. The foremost of them is chelate, and the two follow- 

 ing pairs are each provided with a claw. Of the three pairs of larval 

 appendages Dohrn states that he has satisfied himself that the anterior is 

 innervated by the supra-cesophageal ganglion, and the two posterior by 

 separate nerves coming from two imperfectly united ventral ganglia. The 

 larva is provided with a median eye formed of two coalesced pigment 

 spots, and with a simple stomach. 



The gradual conversion of the larva into the adult takes place by the 

 elongation of the posterior end of the body into a papilla, and the forma- 

 tion there, at a later period, of the anus ; while at the two sides of the 

 anal papilla rudiments of a fresh pair of appendages the first pair of am- 

 bulatory limbs of the adult make their appearance. The three remain- 

 ing pairs of limbs become formed successively as lateral outgrowths, and 

 their development is accomplished in a number of successive ecdyses. As 

 they are formed ca?ca from the stomach become prolonged into them. 

 For each of them there appears a special ganglion. While the above 

 changes are taking place the three pairs of larval appendages undergo con- 

 siderable reduction. The anterior pair singly becomes smaller, the second 

 loses its claw, and the third becomes reduced to a mere stump. In the 

 adult the second pair of appendages becomes enlarged again and forms the 

 so-called palpi, while the third pair develops in the male into the egg- 

 carrying appendages, but is aborted in the female. The first pair form 

 appendages lying parallel to the rostrum, which are sometimes called pedi- 

 palpi and sometimes antenna?. 



The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn, 

 contains rudiments of two pairs of ganglia. 



The larvje of Phoxichilidium are parasitic in various Hydrozoa (Hy- 

 dractinia, etc.). After hatching they crawl into the Hydractinia stock. 

 They are at first provided with the three normal pairs of larval appen- 

 dages. The two hinder of these are soon thrown off, and the posterior 

 part of the trunk, with the four ambulatory appendages belonging to it, 

 becomes gradually developed in a series of moults. The legs, with the 

 exception of the hindermost pair, are fully formed at the first ecdysis after 

 the larva has become free. In the genus Pallene the metamorphosis is ab- 

 breviated, and the young are hatched with the full complement of appendages. 



The position of the Pycnogonida is not as yet satisfactorily settled. 

 The six-legged larva has none of the characteristic features of the Nauplius, 

 except the possession of the same number of appendages. 



The number of appendages (7) of the Pycnogonida does not coincide 

 with that of the Arachnida. On the other hand, the presence of chelate 

 appendages innervated in the adult by the supra-resophageal ganglia rather 



