On the other hand, Cephalodiscida is considered to be a sister subclass of Graptolithina. One of the main differences between these two groups is that Cephalodiscida species are not a colonial organisms. In Cephalodiscida organisms, there is no common canal connecting all zooids. Cephalodiscida zooids have several arms, while Graptolithina zooids have only one pair of arms. Other differences include the type of early development, the gonads, the presence or absence of gill slits, and the size of the zooids. However, in the fossil record where mostly tubaria (tubes) are preserved, it is complicated to distinguish between groups.