Spectra of degrees of some structures

We argue for the existence of structures with the spectrum $\{\mathbf x:\mathbf x\not\le\mathbf a\}$ of degrees, where $\mathbf a$ is an arbitrary low degree. Also it is stated that there exist structures with the spectrum of degrees, $\{\mathbf x:\mathbf x\not\le\mathbf a\}\cup\{\mathbf x:\mathbf x\not\le\mathbf b\}$, for any low degrees $\mathbf a$ and $\mathbf b$.