538 PYCNOGONIDA. 



secondarily adapted to an aquatic respiration, since it is likely (for the 

 reasons already mentioned in connection with the Tracheata) that if Limulus 

 has any affinities with the stock of the Tracheata it is descended from air- 

 breathing forms, and has acquired its aquatic mode of respiration. The 

 anastomosis of the two halves of the generative glands is an Arachnidan 

 character, and the position of the generative openings in Limulus is more 

 like that in the Scorpion than in Crustacea. 



A fuller study of the development would be very likely to throw 

 further light on the affinities of Limulus, and if Packard's view about the 

 nature of the inner egg membrane were to be confirmed, strong evidence 

 would thereby be produced in favour of the Arachnidan affinities. 



(533) A. Dohrn. "Untersuch. lib. Bau u. Entwick. d. Arthropoden (Limulus 

 polyphemus). " Jenaische Zeitschrift, Vol. vi., 1871. 



(534) A. S. Packard. "The development of Limulus polyphemus." Mem. 

 Boston Soc. Nat. History, Vol. n., 1872. 



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 laevis, 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 remaining 

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

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

 are formed caeca 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 considerable 

 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 



