When the ova and spermatozoa are ripe, they are simply shed out into the sea and fertilisation occurs there. The development is described in Chapter XXI. The free-swimming larval period lasts about six weeks.
Having described a single species with some degree of fulness, we must now give some account of the range of variation of structure met with in the group.
Number of Arms.—In the overwhelming majority of Starfish the number of arms is 5, but deviations from this rule are met with not only as individual variations, but as the characteristics of species, genera, and even families.
The number 5 is rarely diminished, but amongst a large collection of specimens of Asterina gibbosa, belonging to the author, some 4-rayed individuals are met with. One species of Culcita, C. tetragona, is normally 4-rayed.
On the other hand the number 5 is often exceeded. The families Heliasteridae and Brisingidae are characterised by possessing numerous (19-25) arms. In the normally 5-rayed family Asteriidae Pycnopodia has 22 arms; and in the Solasteridae the genera Rhipidaster and Solaster are characterised by possessing 8 and 11-15 arms respectively; whilst Korethraster and Peribolaster have only 5. The common Starfish of the Gulf of St. Lawrence, Asterias polaris, is 6-rayed, whilst most of the other species of the same genus are 5-rayed, though 6 rays are often met with as a variation.
In some species the fact that the number of arms exceeds 5 seems to be connected with the power of multiplication by transverse fission. Thus Ludwig[[452]] has shown that in Asterias tenuispina the number of arms is usually 7, but sometimes 5, 6, or 8, and that in most cases the arms are arranged in two groups—one consisting of small arms, the other of large.
Shape.—Apart from the varying number of arms, differences in the shape of the Starfish are due to two circumstances:—
(1) The proportion of breadth to length of arm; and
(2) The amount of adhesion between adjacent arms.
The adhesion can go so far that the animal acquires the shape of a pentagonal disc. This is the case for instance in Culcita. The fact that the body of this animal is really composed of adherent arms is at once made clear when the coelom is opened. This space is found to be divided up by inwardly projecting folds called interradial septa, which are stiffened by calcareous deposits and represent the conjoined adjacent walls of two arms.