420 CROWTHER : BIOLOGY OF SPH^RIUM CORNEUM. 



:the scalloped margins of the gill-plates. The stout cilia have a 

 finger-like movement, especially such as are scattered in spare 

 groups on the inter-marginal areas of the filaments. Teased-off 

 portions of the gill lamellse are singularly vital, behaving like so 

 many protozoans ; a bit with stout cilia on it is marvellously like 

 Euplotes. Isolated ciliated cells are motile so long as life lasts, 

 and then die as is shown by the gradual cessation of ciliary 

 action. This movement may, however, continue for some time; 

 comparatively large pieces teased off gill tissue lived ten hours 

 in .a moist cell. This vitality varies ; cells of the incurrent 

 siphon lived longer than those of the excurrent siphon in teased- 

 off portions of the same animal. Ciliated foot tissue is com- 

 paratively long-lived, but dies somewhat suddenly, and is 

 preceded by a shrinking of the organ to which they belong. 

 The stout cilia of the gills are short-lived, whilst those on the 

 margins are long-lived. 



I believe that in the locality where I gather my specimens, 

 the deposition of ova goes on all the year round. I have taken 

 embryos from the brood pouches from March to January. With 

 the exception of a single specimen, examined in December, all 

 my adult examples have yielded young, an interesting confirm- 

 ation of the herma phroditism of Sphcermm conieum. There are 

 usually six or seven specimens which can be seen with the 

 naked eye in each brood pouched, and I have taken as many 

 as thirty young from within a single animal. The microscope 

 reveals most minute forms in a state of gradual evolution within 

 the gills. Some of the larger specimens are one-sixteenth the 

 size of the parent, and have shells of a compressed sphenoid 

 shape. At the slightest pressure on the gills of the mother- 

 mollusc these bigger embryos cut their way out and then stop, 

 as if anchored by their byssal threads, and forced separation 

 from the parent requires a pull with the teasing needle. 



As the young are found of all sizes in the brood pouches, 

 there is probably in this form a prolonged deposition of ova; 

 in localities where the water is tempered by meteoric changes 



J.C., vii., Oct. 1S94. 



