54 ZOOPHYTES. 



secretions are nearly or quite absent immediately about each polyp- 

 pore, in order that these parts may be flexible to admit of the animal's 



contracting and expanding itself; 

 elsewhere they are more or less 

 abundant, according to the species. 

 In the genus Tubipora, the spe- 

 cies form a thin calcareous tube 

 for each polyp, smooth within, as 

 well as without, yet perforated by 

 minute pores. 



48. Among the Actinaria, the principle, that the calcareous and 

 fleshy lamellae have a general correspondence in number, leads 

 to important distinctions in the coralla. Madrepores, on this prin- 

 ciple, can never have over twelve rays to their cells, and the number 

 is usually from six to twelve; while the Caryophyllia and Astreea 

 tribes have an indefinite number. The two tribes just mentioned are 

 distinguished in their coralla by another character depending on the 

 structure of the animals, as explained in 42. The lamellae of the 

 stars in an Astrsea, and the allied corals, extend through the intersti- 

 tial spaces between the cells, striating lamellately the surface ; while 

 in the recent species of the Caryophyllia group (and also the Madre- 

 pores), these spaces are smooth, granulous, or irregularly porous, 

 instead of lamello-striate. Besides, in the former, the calcareous 

 lamellae are united by numerous transverse dissepiments, which, 

 when oblique, as in plate 11, figure 4c (showing a vertical section of 

 an Astrasa), they cause the star in a transverse section to be divided 

 into numerous cellules (fig. 4d) ; but, if horizontal, or nearly so, as in 

 plate 10, figure 2, they do not show their edges in a transverse view, 

 and the star appears simple. In the recent Caryophyllidse, there are 

 seldom any cross dissepiments, and in the few cases in which they 

 are observed, they are distant, being separated by longer intervals 

 than the breadth of a lamella. 



49. Foot-secretions. The foot-secretions appear to be entirely inde- 

 pendent of the tissue-secretions. The former are often horny, when 

 the latter are calcareous, and when they occur together they consti- 

 tute separable layers, one enveloping the other. 



The united polyps of a branch have their mouths opening outward 

 on every side, while the bases are directed inward towards a common 

 central or axial line. The simultaneous secretions of the bases, there- ' 

 fore, must necessarily produce a solid axis to the branch. 



