140 



PHYSIOLOGY. 



Fig. 57. 



Part of the umbrella-margin of Tyaropsis scotica. 



c, lithocyst ; 

 endoderm ; 



a. Circular canal ; b, b, marjrin.il tentacles ; 

 d, ectoderm of distal side of circular canal ; ( 

 f, projection of the ectoderm, lying at the inner side of the 

 lithocyst ; ff, ocelliform spot, imbedded in the endoderm of 

 the circular canal. 



umbrella. It consists of a transparent, mostly spherical capsule, williin which are contained one 

 or more transparent refractile concretions generally of a spherical or oval form. In Cunina alone 



among the Hydroida the concretions are, ac- 

 cording to Haeckel, in the form of ciystals, the 

 usual condition of the analogous bodies in the 

 Biscophora or steganophthalmic medusse. In 

 most cases there is but one of these bodies in each 

 capsule. Sometimes, however, they are more 

 numerous. In Tuna Bairdii the number varies 

 from four to twenty in different lithocysts of the 

 same specimen. In T^arojjsis they are arranged 

 in a regular crescent parallel to the distal wall of 

 the capsule, as Agassiz has pointed out in a 

 North American species of this genus, and as I 

 have myself found in a Tyaropsis from the 

 Scottish coast (woodcut, fig. 57), in which I have 

 counted about thirteen concretions in each capsule. 

 The concretions offer resistance to pressure ; and, according to Gegeubaur, would seem to 

 consist of carbonate of lime in an organic basis, which retains the form of the original 

 body after the mineral matter has been removed by acid. The mineral constituent is, according 

 to Haeckel, a phosphate of lime in the Geryonida. At least, it here dissolves in acids without 

 effervescence. The concretions, in every case, are quite motionless, and never lie free in the 

 capsule. 



A careful study of the lithocysts in two very common hydroids — Campanularia Jo.h/sfoni nr\d 

 Ohelia [Laoniedea) geniculala — has rendered apparent the following facts, which may probably be 

 regarded as representing, in all essential points, the usual structure of these bodies among the 

 Hydroida. 



In Campcamlaria Johnsloni there are eight lithocysts which alternate with the eight marginal 

 tentacles of the medusa ; each lithocyst (woodcut, fig. 58/<) is immersed for a little way in the 

 cord-like structure, which forms the extreme margin of the umbrella, and which sends a very 

 delicate extension of its substance over the whole of the free surface of the lithocyst. It 

 consists of a spiierical transparent and structureless vesicle or capsule, the greater part of whose 

 cavity is occupied by a soft spherical pulp, in whose distal pole, or that opposite to the point 

 of attachment of the vesicle, there exists a deep well-defined excavation ; and within this, but 

 not entirely filling it, is the spherical highly refractile concretion. In the pulp itself I could 

 detect no trace of structure, but when seen in profile it has a slightly wavy outline, possibly occa- 

 sioned by a special layer which intervenes between it and the walls of the capsule. Its surface 

 is marked by twelve or fifteen delicate striae, which take a meridional course at exactly equal 

 distances from one another. Towards the distal pole they all terminate distinctly on the margin 

 of the excavation, and may be thence traced to within a short distance of the opposite pole, 

 though I have never been able to follow them exactly to it. The striae generally appear 

 light-coloured when contrasted with the darker intervening spaces. It is often difficult 

 to detect any trace of them, but with a high power and carefully adjusted illumination 



