PCECILOPODA. 447 



Three weeks after hatching an ecdysis takes place, and the larva passes 

 from a trilobite into a liinuloid form. The segmentation of the abdomen 

 has become much less obvious, and this part of the embryo closely resem- 

 bles its permanent form. The caudal spine is longer, but is still relatively 

 short. A fourth pair of abdominal appendages is established, and the first 

 pair have partially coalesced, while the second and third pairs have become 

 jointed, their outer ram us containing four and their inner three joints. 

 Additional gill-lamellse attached to the two basal joints of the second and 

 third abdominal appendages have appeared. 



The further changes are not of great importance. They are effected in 

 a series of successive moults. The young larvae swim actively at the 

 surface. 



Our, in many respects, imperfect knowledge of the development of 

 Limulus is not sufficient to shew whether it is more closely related to the 

 Crustacea or to the Arachnida, or is an independent phylum. 



The somewhat Crustacean character of biramous abdominal feet, etc. 

 is not to be denied, but at the same time the characters of the embryo 

 appear to me to be decidedly more arachnidan than crustacean. The 

 embryo, when the appendages are first formed, has a decidedly arach- 

 nidau facies. It will be remembered that when the limbs are first formed 

 they are all post-oral. They resemble in this respect the limbs of the 

 Arachnida, and it seems to be probable that the anterior pair is equivalent 

 to the chelicerae of Arachnida, which, as shewn in a previous section, are 

 really post-oral appendages in no way homologous with antennae 1 . 



The six thoracic appendages may thus be compared with the six 

 Arachnidan appendages ; which they resemble in their relation to the 

 mouth, their basal cutting blades, etc. 



The existence of abdominal appendages behind the six cephalothoracic 

 does not militate against the Arachnidan affinities of Limulus, because in 

 the Arachnida rudimentary abdominal appendages are always present in 

 the embryo. The character of the abdominal appendages is probably 

 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. 



1 Dohrn believes that he has succeeded in shewing that the first pair of appendages 

 of Linmlus is innervated in the embryo from the supra-cesophageal ganglia. His 

 observations do not appear to me conclusive, and, arguing from what we know of the 

 development of the Arachnida, the iunervation of these appendages in the adult can be 

 of no morphological importance. 



