850 



SYMMETRY. 



seum of the College of Surgeons (No. 205.), 

 there is a portion^of a tcenia lata, which ex- 

 hibits a monstrosity very interesting in its 

 bearing on the question of the individuality 

 of half segments (p. 845.). On one side, 

 which for distinction we will call right, there 

 are three half segments, the middle one of 

 which unites with two half segments of the 

 left side, leaving the upper and lower of the 

 three right halves isolated and independent. 

 One of these contains a generative apparatus. 

 Mollusca. In this sub-kingdom absence 

 of symmetry seems to be the rule its pre- 

 sence the exception. There are great prac- 

 tical difficulties in the way of finding an ab- 

 stract notion or type of a mollusk such as 

 have been found for the Vertebrata and Arti- 

 culata. If, however, such type be symme- 

 trical, that symmetry is departed from much 

 oftener than it is preserved. There is no 

 appearance in any mollusk of a serial re- 

 petition of parts nothing like serial ho- 

 mology, except in the Chitons, whose shell 

 consists of a number of similar symmetrical 

 transverse bands. The highest class of mol- 

 lusks, the Cephalopoda, are symmetrical, and 

 when they inhabit shells, as the Argonaut 

 and pearly Nautilus, their shells are symme- 

 trical. But the enormous number of species 

 comprised in the classes Gasteropoda and 

 Acephala are nearly all un symmetrical. The 

 slugs are the most symmetrical of the Gaste- 

 ropoda in their external form, but here the 

 air-orifice, for instance, is on one side. 



Radiata. For convenience of illustration, 

 I select from among the animals composing 

 this sub-kingdom the common star-fish. (Figs. 

 16, 22, &c. Vol. I. Art. ECHINODERMATA.) 

 This animal presents to our view a flattened 

 form with five exactly similar rays or arms ra- 

 diating from the centre, where is posited the 

 mouth. Around the.mouth is a nervous circle 

 consisting of a number of ganglia corresponding 

 to the arms, connected with each other by 

 intercommunicating cords. Each arm is 

 symmetrical in itself. Now this figure may 

 be divided into two symmetrical halves by a 

 line drawn across it in either of five different 

 diameters. What is to decide which of these 

 is to be regarded as the mesial line? For- 

 tunately there is, on the side which is the 

 reverse to that on which the oral orifice is 

 placed, a peculiar spot, the remains of an 

 embryonic structure, which is not in the 

 centre, and will therefore serve for a datum. 

 In certain other echinoderms the anus is situ- 

 ated eccentrically, in which case it also may 

 be taken as a datum. But still a line drawn 

 through either of these, the number of the 

 arms not being an even number, will divide 

 one of them in its middle on one side, and 

 pass through the interspace between two of 

 them on the other. This, as the arms are 

 exact repetitions of one another, seems an 

 unnatural and arbitrary proceeding. Still, 

 though it possesses no natural middle line, 

 and consequently is not bilateral, yet is the 

 star-fish a symmetrical animal, for the idea 

 of bilateralism is by no means included in our 

 definition of symmetry. 



Whenever a number of exactly similar parts 

 symmetrical in themselves are arranged around 

 a centre, whether their number be two, as in 

 bilateral forms, or five, as in the star-fish, the 

 whole figure is symmetrical. Abandoning 

 then the idea of bilateralism, we may look 

 upon the star-fish as composed of five repe- 

 titions arranged around a centre. Regarding 

 it in this manner, we are able to institute a 

 comparison between the form of the star-fish 

 and that of the segmented vertebrates and 

 articulates. The star-fish may be regarded as 

 analogous to one segment of a vertebrate or 

 articulate an animal with one vertebra. 



I use the term " analogous," and not 

 " homologous," because the relation called 

 special homology cannot be demonstrated, in 

 any instance, between animals belonging to 

 different sub-kingdoms. The head, the legs, 

 the brain of an articulate, are only function- 

 ally the head, legs, or brain. They perform 

 the same function as, but they cannot be shown 

 to be homologous with, the head, legs, or brain 

 of a vertebrate. Indeed I have long held that 

 the sub-kingdoms should be limited by refer- 

 ence to special homology : all animals among 

 which homologies can be pointed out which 

 all conform to the same type should be 

 grouped together to form a sub-kingdom. 



The view of the analogy of a radiate animal 

 to a vertebrate or articulate animal just given, 

 is considerably strengthened by the manner of 

 development of the common medusa. The 

 larva of the medusa is a polypiform living 

 thing, anchored by one end and tentaculated 

 at the other. This, after a while, becomes 

 marked with numerous constrictions, like a 

 segmented animal, which constrictions become 

 more and more deep until they completely 

 divide the quondam polypiform being into a 

 number of pieces, each of which becomes a 

 perfect medusa. Here is a segmented animal, 

 each of whose joints becomes an independent 

 radiate individual ; ergo, each individual is 

 analogous to one segment of a vertebrate or 

 articulate. 



Towards a different view tends the fact, that 

 though five, or a multiple of five, is usually 

 the number of the rays of the star-fish, yet 

 there are some members of the same group 

 the number of whose arms are neither the one 

 nor the other; for instance, there are eleven 

 and twelve-armed asterias. Now there is such 

 perfect constancy in the number of the parts 

 of vertebrate and articulate segments, that that 

 constancy seems an integral element of the 

 idea of an archetype. There is, however, no 

 constancy in the number of the vertebrate or 

 articulate segments that go to constitute an 

 animal, and therefore some may regard each 

 arm of an asteria as analogous to a vertebra ; 

 the common star-fish, as composed of five 

 analogues of vertebral archetypes arranged in 

 a circle, as a segmented animal bent upon 

 itself, with its anterior and posterior extremities 

 adherent to one another. Which of these two 

 views is the correct one whether either is 

 correct can be decided then only when the 

 true import of serial homology and of sym- 



