In They have it, have is a full verb, but in They have done it, it's an auxiliary verb.

In this case, then, the infinitival morpheme functions more locally to simply derank the verb in the purpose clause.

As such, Megalograptus has been considered to represent the plesiomorphic condition within eurypterids for a number of characters, including the occurrence of the opisthosomal preabdominal/postabdominal constriction at the sixth and seventh segment as opposed to the seventh and eighth [ 5 ], the possession of an extra podomere on prosomal appendages II–V [ 72 ], and the morphology of the prosomal ventral plates [ 37 ].

Here, we microengineered a three-dimensional (3D) model of the human blood-brain barrier (BBB) within a microfluidic chip by creating a cylindrical collagen gel containing a central hollow lumen inside a microchannel, culturing primary human brain microvascular endothelial cells on the gel’s inner surface, and flowing medium through the lumen.