188 ONYCHOPHORA. 



more striking. The prominences now under consideration are, according to 

 v. Kennel, retained for a long time, and have perhaps escaped observation in 

 later stages owing to the development of papilla-like prominences of the integu- 

 ment, such as occur in great numbers in the adult. St. Remy (No. 8) describes 

 and illustrates a paired ganglionic swelling on the brain of the adult Peripatus 

 ("formation de nature inconnue"), which in position corresponds to the two 

 prominences on the head of the embryo, and which might well be regarded as 

 the lobe of a primary tentacle-nerve. In the posthumous works of Balfour 

 also, similar structures are described as pairs of nerves (running to various points 

 of the dorsal surface), and of these one might belong to such a sensory organ. 



We cannot refrain, in this connection, from calling attention to the sensory 

 organs found in the cephalic region in many Myriopoda (e.g. , Lithobius, 

 Polyxenus, and Glomeris), the innervation of which is said to take place from 

 the "optic thalamus" (Tomosvaky, No. 22, p. 760). We must however impress 

 upon the reader that the actual material required for a successful comparison of 

 this peculiar sensory organ with the frontal organ of the Crustacea, or with the 

 still insufficiently investigated prominences of Peripatus has not yet been 

 obtained. 



In the younger embryos of Peripatus (such as that illustrated in Fig. 91 A), 

 the change of position of the antennae, if these are considered as proceeding 

 from limbs, is not very marked, especially as compared with the corresponding 

 change that takes place in the Insecta. The position of the eyes in Peripatus 

 is less easily reconciled with this view. The eyes lie further back than the 

 antennae, close to that part of the brain which must be derived from the first 

 (primary) trunk-segment. The eyes, however, may well be ascribed to the 

 primary cephalic segment, especially as, in Peripatus, they agree with the eyes 

 of the Annelida rather than with those of the Arthropoda. This can only be 

 explained as having been brought about by the shifting of the various parts 

 which participate in the formation of the head. 



3. The Formation of the Organs. 

 The Ectodermal Structures. 

 The Integument. 

 The ectoderm forms a single layer of cubical cells over the greater 

 part of the body of the embryo. In P. capensis these cells, especially 

 on the dorsal surface, are said by Sedgwick not to be sharply 

 demarcated externally, and to exhibit a spongy structure. Sedgwick 

 on this account ascribes to them the capacity for absorbing fluid 

 nourishment, and believes that the placenta described by v. Kennel 

 might arise as a more specialised ectodermal organ for taking in 

 nourishment. The changes undergone by the ectoderm when trans- 

 formed into the adult integument are not very important. The 

 delicate cuticle which occurs in the adult is secreted externally. 

 At some points, e.g., on the ventral side of the limbs, the ectoderm 

 becomes multilaminar and here secretes a thicker layer of chitin, 

 and this is also the case at the distal extremities of the limbs where 

 the claws are formed. 



