728 TRANSACTIONS OF SECTION D. 



the genus to be a natural one. The genus as a whole possesses an almost bewilder-t 

 ing variety in the form, position, and size of the avicularia, among which, how- 

 ever, we may distinguish the following kinds : (i) The suboral avicularium, 

 closely related to the orifice and usually termed ' labial,' because it occurs on 

 what may be described as the lower lip ; (ii) frontal avicularia, on some part of 

 that surface of the zooscium which bears the orifice ; (iii) basal or dorsal 

 avicularia, on the backs of the branches ; (iv) fenestral avicularia, which guard 

 the edges of the fenestraj or meshes of the colony. 



In many of the species of this large genus the suboral avicularia are of the 

 small rounded type. In other species they are small and pointed, with an acute 

 mandible ; while others are distinguished by possessing suboral avicularia that may 

 ba described as gigantic. 



Among the frontal avicularia similar diflferences exist. In one case that has 

 come under my observation a remarkable variation of this kind is found within 

 the limits of a single species. Remembering the great difficulty there often is in 

 arriving at certainty with regard to the limits of the species in the genus under 

 consideration, I wish to emphasise the fact that this instance is taken from 

 Retcjwra phanicea, a form that not only has well-marked specific characters 

 of the ordinary kind, but is remarkable in having a beautiful carmine-red or violet 

 colour, a respect in which it differs from most of its nearest allies. The frontal 

 avicularia of this species are usually of the pointed type, but in the variety in 

 question — a colony from Torres Straits — they are, so far as I have been able to 

 ascertain,' all of the rounded kind. 



The fenestral avicularia show a similar behaviour. In South Australian 

 waters there are a number of forms which are regarded as varieties of Retepora 

 mo7iilifera. In the form known as var. munita there is usually a suprafenestral 

 avicularium of large size, distinguished by having a rounded mandible, which is 

 a good deal broader than it is long.'- In another form of the same species, dis- 

 tinguished by MacGillivray as var. acutirostris, the wiwztto-avicularium may 

 either occur as such in some of the fenestrse, or be replaced in others by a large 

 avicularium of the typical pointed form. 



In other .species a gigantic infrafenestral avicularium commonly occurs, but 

 while these structures are found in a considerable proportion of the fenestrse of 

 some colonies they appear to be completely absent in other colonies. In this 

 series of cases, which is well illustrated by Retepora phcenicea, I think there is 

 clear evidence that different colonies, from the same locality and belonging to the 

 same species, may show the two conditions of presence and absence respectively 

 of fenestral avicularia. According to the ordinary criteria by which species of 

 Polyzoa are discriminated, it might be necessary to place these in different species — 

 a result which is not supported by other evidence. I think we must conclude 

 that a species may have the faculty of entirely dropping out some complete series 

 of organs, like certain kinds of avicularia. The Alendelian principle may here 

 come to our aid by showing the theoretical possibility of having the two con- 

 ditions represented in a series of colonies of identical parentage. If this should 

 really be the explanation of the facts, it should occasion no surprise if some mem- 

 bers of the immediate progeny of a colony in which a certain type of avicularium 

 is absent should be found to be provided with a complete armature of these 

 appendages. 



The cases so far considered may conceivably be explained on ordinary Men- 

 delian lines by assuming that an entire colony is homozygous or heterozygous 

 with regard to particular characters. Remembering that the so-called ancestrula, 



' It may be noted that it is extremely difficult and often impossible to make a 

 study of every part of a large and irregular Retepora sufficiently exhaustive to 

 justify one in asserting positively that all parts are identical in respect of their 

 avicularia. 



^ This characteristic ?nMnite-avIcularium is probably merely an enlarged form of 

 the small circular type of avicularium met with as labial avicularia and in other 

 positions in roany species, 



