THE ANTHOZOA 41 



the mouth, and by the pores at the tips of the tentacles. In the 

 family Sagartidae there are in addition perforations in the lower 

 third of the body wall called cinclides through which the acontia 

 are protruded. In S. parasitica each cinclis is placed on the summit 

 of one of the warty tubercles scattered over that region of the 

 body. The histology of the Actiniae has been studied with great 

 care by 0. and R. Hertwig (40), to whose work the reader should 

 refer for details. The general features of the histology have 

 already been given on p. 9. The general anatomical features of 

 an Actinian zooid may be studied in Fig. XIX. 1. 2 shows the 

 structure of a mesentery and the arrangement of its musculature. 

 3 and 4 show the order and relations of the mesenteries. 



The mesenteries are the most important organs of the 

 Zoantharian zooid, and it is of great importance that their arrange- 

 ment and order of succession should be thoroughly understood, 

 since they afford the only characters which have hitherto been 

 found to be of definite taxonomic value. The arrangement of the 

 mesenteries in a typical Actinian is shown in Fig. XIX. 4. As 

 has already been stated, they are arranged in couples, the muscle 

 banners of each couple are turned towards one another, except in 

 the two couples of directive mesenteries (dd) whose muscle banners 

 face outwards. The following points must be noted over and 

 above the situation of the longitudinal muscles and the position 

 of the directive mesenteries : 



(a) The mesenteries are arranged in cycles : six couples in the 

 first cycle, six couples in the second, twelve couples in the third, 

 twenty-four in the fourth, and so on. Mesenteries of the same 

 cycle are of the same size and (with the exceptions mentioned 

 hereafter) were formed at the same time. The mesenteries first 

 formed, the primaries, are as a rule the largest ; the secondaries 

 are next in size ; the tertiaries smaller than the secondaries, and 

 so forth. The two couples of directive mesenteries belong to the 

 first cycle. 



(b) Any two mesenteries forming a couple belong to the same 

 cycle, and are therefore of the same size. The two mesenteries 

 forming a couple are separated by a narrow space, an entocoele ; the 

 two mesenteries of adjacent couples are separated by a wider space, 

 an exocoele. 



(c) With the exception of the directives the longitudinal 

 muscles of the mesenteries are always entocoelic, the transverse 

 muscles exocoelic. 



(d) New couples of mesenteries always take their origin in the 

 exocoeles, never in the entocoeles. 



It is common to find six couples of primary mesenteries in the 

 Zoantharia. So commonly does this number occur that at one 

 time the Zoantharia were named the Hexactiniae, in opposition to 



