EARLY STAGES OF TEREBRATULINA. 35 



marked. The same projection is represented in the mouth of Thecidlum Mediterraneum, 

 as figured by Lacaze-Duthiers. A singular lunate groove, running parallel with the free 

 lip, is indicated in the figures, which may be called the oral groove (og). It seems more 

 like a wrinkle caused by the expansion of the free lip, though its limits are well defined 

 and the furrow is quite deep. 



The independent movements of the cirri are shown in fig. 37 ; these are just as pliant 

 in their movements as in those of the extreme young, and at all times recall the motions 

 of the cirri in the Polyzoa. 



Structure of the Shell. In the younger stages of the shell the peculiar scale-like struc- 

 ture may be studied to the best advantage. The scales are few in number, but slightly 

 overlapping, and form a layer quite distinct from the outer layer, which appears to be 

 homogeneous, save the concentric lines of growth appearing like rows of oblong and flat- 

 tened nodules. The terminal portions of the pallial caeca within or upon this outer layer, 

 are brown in color and distinctly granulated. I failed to make out anything satisfactory 

 regarding the character of these peculiar parts. In a few doubtful cases I thought I saw 

 one or two tubules radiating from the terminal ends of the caeca, and in fig. 38, A, one 

 observation is represented. It will be seen by this that the tubules radiate from the 

 largest diameter of the caeca, and not from the periphery of the granulated disk, and this 

 is in accordance with the admirable observations made by King. 1 The scales do not 

 appear to encroach upon the walls of the caeca. 



Fig. 38 shows a small fragment of the shell at an early stage ; portions of five scales 

 are here seen forming the inner layer of the shell. In the earlier stages, when they are 

 large and few in number, the scales are flat ; now and then showing a raised line caused 

 by the encroachment of two superincumbent scales, between which the under scale is 

 closely adjusted. This may be seen in fig. 39, and better in fig. 40, where the ends of 

 the scales are broken, and show in section ; also in fig. 41, where, in a later stage, they 

 become more numerous ; here the terminal ends of five scales are shown in juxtaposition. 



In fig. 42 a portion of the anterior margin of one valve is shown. The newly formed 

 scales are just indicated near the border, beyond which the outer layer projects. 



Fig. 43 represents two scales from a stage still more advanced ; here the numerous 

 lines upon the scales indicate their closer aggregation. As the shell increases in age, the 

 scales become more attenuated, and now the scale is diverted wherever it encounters a 

 csecal process, and forms a yoke or loop around it. 



In fig. 15, plate i, a few spiculas at the base of the cirri mark the commencement of 

 the calcareous loop. At a later stage the spiculae increase rapidly in number, and finally 

 anastomose, appearing as in fig. 44. The scales composing the base of the crura are 

 acicular in shape, and run parallel with the crural axis. In adult specimens the scales of 

 the crural supports resemble acicular crystals, and when boiled in a solution of caustic 

 potash, glisten and separate, resembling the fibres of asbestos (fig. 47). 



Fig. 46 represents the crural process of one side, taken from a specimen less than an 

 eighth of an inch in length. It exhibits the first stages of the loop. The acicular scales 

 are shown at the base, while the remaining portion is made up of scales similar to those 



1 Prof. W. King, on the Histology of the Test of the Academy, vol. xxiv, pt. xi, 1869. 

 Class Palliobranchiata. Transactions of the Koyal Irish 



