CHAPTER XXII. 



PENTASTOMIDAE. 



Our knowledge of the Pentastomidae still rests principally on 

 Leuckart's observations, supplemented by a few smaller treatises, 

 and recently confirmed and amplified by Stiles. 



1. Embryonic Development. 



The eggs of Pentastomum are surrounded by two envelopes (Fig. 

 58 A and B, h). The early embryonic development takes place 

 gradually as the ovum passes down the uterus. Cleavage is total 

 (Leuckart, Macalister). The egg breaks up into a number of cells 

 of about equal size, the further fate of which could not be ascer- 

 tained. Macalister describes the formation of a blastoderm and a 

 germ-band, but his statements are not conclusive. According to 

 Leuckart, a germ-band is not formed. The embryo secretes a 

 surface cuticle at an early stage, a disc-like thickening appearing 

 on the dorsal surface of this cuticle. When the cuticle detaches 

 itself from the embryo and forms a third envelope to the latter 

 (Fig. 58 A and B, eh), it remains connected at this thickened disc 

 (dorsal cone, rz). The chitinous integument also, which is now 

 secreted as a covering for the embryo, is correspondingly thickened 

 at this spot, and takes the form of a pit-like depression. The "dorsal 

 cone," which at first connects these two chitinous thickenings, 

 becomes constricted and broken through, but a trace of it is left 

 attached to the embryo; this, in P. taenioides, is shaped like a raised 

 cross situated in a cup-shaped groove (Fig. 58 B and C, rk). The 

 remainder of the" dorsal cone" is retained on the detached integu- 

 ment as a circular thickening (the so-called facet, Fig. 58 B, /). 

 This structure recalls the ruicropyle or the dorsal organ of the 

 Crustacea, with which Leuckart has compared it. 



A certain external similarity in structure is found between the so-called 

 primitive tracheae of the Acarina and the dorsal cone of the Pentastomum ; but 

 these "tracheae" are paired and He ventrally, so that there is no real agreement 

 between them. 



K 



