INTRODUCTION. 



25 



It will be noticed that the cups for the tube-feet are now much smaller, and 

 there can be no doubt that the podia are losing (or have lost) their " walking " 

 function and become reduced in size as in Recent Ophiuroidea. Further advances 

 towards the Ophiuroid structure are to be seen in — 



(a) The diminution in importance of the adambulacral longitudinal muscles as 

 shown by the faint concavity on the faces of the adambulacra. 



{!)) The enclosure of the branch radial water-vascular canals in the substance 

 of the ossicle. 



(c) The presence of Ophiuroid spines on the adambulacralia (Text- fig. 22). 



(d) The hollow ridges for the articulation of the ambulacralia with the adam- 

 bulacralia. 



There are forwardly projecting pegs which fit on distal processes of the 

 preceding ambulacralia. These pegs are hidden in oral view by the large articular 

 faces for the longitudinal muscles. They can, however, be seen in side view 

 after removal of the adambulacralia (Text-fig. 24), or when the arm is slightly 



PROXIMAL. 



Text-fig. 24. — Side view of two isolated ambulacralia of Aspidosoma grayx. 3f.(2), depression for dorsal longi- 

 tudinal muscles; 3f.(5), depression for ventral longitudinal muscles ; e. <./., opening of branch canal to 

 tube-foot; Ai\ p., articulating peg. 



bent they may be seen on the abactinal surface deep down between the ossicles. 

 The abactinal integument contained few or no calcifications. 



The Structure of the Arm of Lapivortliura miltoni. 



An account of the structure of the arm in Lcipioorthura has recently been given 

 by Sollas and Sollas {7], pp. 216, 217). There ie little new to add to this account. 

 Orally (PI. I, fig. 10), the ambulacralia are very similar to those of Asjudosoma, 

 except that they are not alternate, but opposite. There is the same large proximal 

 depression for the ventral longitudinal " wriggling " muscles, and in both forms 

 the ossicular canal is large and the cup for the tube-feet much reduced. The 

 branches from the radial water-vascular vessel to the tube-feet pierce through the 

 walls of the ossicular canal. Sollas describes this branch-canal as a deep groove 

 with thickened edges. According to my observations the branch canal may be 

 completely enclosed in the substance of the ossicle as in Aspidosoma. The 

 adambulacralia show even a further transition towards the stage met with in 

 modern Ophiuroidea than do those of Aspidosoma. In Aspidosoma these plates are 



4 



