422 Dr. E. S. Morse on the Early Stages 



fig. 22 ; these are just as pliant in their movements as 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 structure may be studied to the best 

 advantage. The scales are few in number, but slightly over- 

 lapping, 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 flattened nodules. 

 The terminal portions of the pallial ceeca within or upon this 

 outer layer are brown in colour and distinctly granulated. I 

 failed to make out any thing satisfactory regarding the charac- 

 ter of these peculiar parts. In a few doubtful cases I thought 

 I saw one or two tubules radiating from the terminal ends of 

 the cgeca ; and in fig. 23 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 granu- 

 lated disk ; and this is in accordance with the admirable 

 observations made by King *. The scales do not appear to 

 encroach upon the walls of the ceeca. 



Fig. 23 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 superincum- 

 bent scales, between which the under scale is closely adjusted. 

 This may be seen in fig. 24, where the end of a scale is 

 broken and shown in section. As the shell increases in age, 

 the scales become more attenuated; and now the scale is 

 diverted wherever it encounters a caecal process, and forms 

 a yoke or loop around it. 



In the young state a few spicula at the base of the cirri 

 mark the commencement of the calcareous loop. At a later 

 stage the spicula increase rapidly in number, and finally 

 anastomose, appearing as in fig. 25. 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. 27 represents the crural process of one side, taken from 

 a specimen less than an eighth of an inch in length ; it exhi- 

 bits the first stages of the loop. The acicular scales are shown 



* Prof. W. King " On the Histology of the Test of the Class Pallio- 

 branchiata," Transactions of the Royal Irish Academy, 1869, vol. xxiv. 

 pt. 11, 



