44 THE APODOUS HOLOTHTJEIANS 



generally wanting in other genera. In most Synaptids the calcareous particles 

 are not of sufficient i^rominence to affect tlie appearance of the body surface, but 

 in those species in which the anchors are more than 250 /x in length, or where 

 the calcareous bodies are accumulated in special heaps (as the so-called "wheel- 

 papillai" of Chiridota), the surface may become rough and even very prickly, 

 not only to touch but to sight. The degree of roughness depends to no little ex- 

 tent on the amount of contraction of the circular muscles of the body-wall, and 

 is almost always more noticeable in preserved than in living specimens. In 

 Trochoderma the body-wall is firm and stiff from the crowded calcareous de- 

 posits. 



Tentacles. — The number of tentacles varies from 10 to 27, but it must be 

 borne in mind that the number in the young is at first five, and that thereafter 

 the number varies with the age of the animal, certainly until sexual maturity is 

 reached, and in some eases possibly thereafter. In some species there is a dis- 

 tinct 7-tentacled stage, while in others the second quintet of tentacles appears 

 simultaneously, and we have a 10-tentacled larva. After that it is probable the 

 tentacles appear singly or in pairs ; they certainly do in the 12-tentacled spe- 

 cies, whose development is known, but we have no knowledge of the develop- 

 ment of any species with more than 12 tentacles. Now these facts have an im- 

 portant bearing on what we are to consider the normal number of tentacles 

 in a given species. Where a large numljer of mature individuals have been ex- 

 amined, the normal number of tentacles is not difficult to determine, but where 

 only a few individuals, and some or all of those obviously immature, have been 

 available, it is clear that we cannot decide positively on the number for that 

 species. Unfortunately, comparatively few species are sufficiently well known to 

 make our knowledge on this point satisfactory. Although 10 tentacles are char- 

 acteristic of Rhabdomolgus, Trochoderma, and Trochodota, and of some species 

 of Synaptula, Leptosynapta, Protankyra, Tamiogyrus, and Myriotrochus, few 

 of these latter forms are sufficiently well known to put the matter beyond dis- 

 pute. One species of Labidoplax {hushii) has constantly only 11 tentacles, and 

 since many specimens have been examined, this is probably the normal number. 

 Many Synaptids have the number of tentacles constantly 12, and individual 

 specimens of these species not infrequently have 13. Several species have been 

 described as having 13 tentacles, but all are known from only a few specimens, 

 and it is quite possible that 13 is not the normal number. Many species have 15 

 tentacles, and individuals of these may have 13, 14, or 16 instead of the normal 

 number. Several species have been described as having 16, 17, 18, or 19 tenta- 

 cles, but all are East Indian forms, and not one can be said to be satisfactorily 

 known. It is very probable that the normal number for these species will be 

 found to be 20 or possibly 18. It is rather remarkable that not one authentic 

 species is known with 20 tentacles, though one or two have been so described. 



