CTENIDIAL NUMBER IN RELATION TO SIZE IN CERTAIN CHITONS, WITH A DISCUSSION OF ITS PHYLETIC SIGNIFICANCE

Abstract

1. The discovery and description of Neopilina has reopened discussion of possible metameric segmentation in the primitive molluscs. This has involved reconsideration of the multiplied organ systems in chitons, which some authorities regard as metameric in origin. The length-weight relationship was determined and ctenidial numbers counted for populations of two species of chitons: Chaetopleura apiculata from Cape Cod, and Lepidochitona cinereus from the North Sea coast of Scotland. Less detailed observations were made on other species of chitons and on other organ systems. Growth continues in adult chitons, together with the addition of ctenidia anteriorly, so that there is a correlation between gill-number and adult size. Chaetopleura apiculata adults, weighing from 38 to 1324 mg., had gill-numbers ranging from 17 to 25 per side. Lepidochitona cinereus adults, weighing from 6 to 340 mg., had gill-numbers ranging from 11 to 20 per side. Asymmetry in ctenidial numbers between the left and right sides of single specimens occurred in 19.5% of Chaetopleura apiculata, and in 48.4% of Lepidochitona cinereus. Qualitative asymmetry was even more extensive in the populations studied. Since such marked ctenidial asymmetries as reported above occur, and since individual gills are added independently on either side, the ctenidia of chitons cannot be paired structures. The organization of certain other replicated structures (including shell-plates, auriculoventricular connections, and renal lobes) is considered more briefly, drawing on earlier pertinent work, and leads to the obvious result that individual ctenidia cannot be related to any other replicated structure— still less allocated to specific "segments." 2. These findings are discussed in relation to the alleged metamerism of chitons, and it is also concluded that such replicated organs cannot represent a simplification of a more extensive metamerism. Further, it is shown that it is possible to criticize the concept of metamerism as applied to the described structures of Neopilina. The phyletic significance of this is explored, and it is concluded that evidences for a connection between the primitive molluscs and the turbellarian-rhynchocoele phyla are better than are those for metamerism and an annelid-arthropod connection. A model ancestral mollusc with a four-fold basic organization (e.g., four ctenidia, four auricles, four renal organs, etc.) is proposed. It is stressed that, while any model of an ancestral mollusc is highly speculative, the evidences against metamerism in chitons (and probably in all primitive molluscs) are overwhelmingly strong.